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of two adjacent Mastomys natalensis (Rodentia:
Muridae) populations
Aude Lalis, Michel Baylac, Jean Cosson, Rhodes Makundi, Robert S Machang’U, Christiane Denys
To cite this version:
Aude Lalis, Michel Baylac, Jean Cosson, Rhodes Makundi, Robert S Machang’U, et al.. Cranial mor- phometric and fine scale genetic variability of two adjacent Mastomys natalensis (Rodentia: Muridae) populations. Acta Theriologica, Springer Berlin Heidelberg, 2009, 54 (2), pp.171-181. �hal-02978289�
In tro duc tion
The Af ri can en demic ge nus Mastomys, Thomas (1915) or multimammate rat (Rodentia,
M u r i d a e ) i s r e s p o n s i b l e f o r i m p o r t a n t ag ri cul ture and pub lic health prob lems by caus - ing sig nif i cant dam ages to cul ti vated fields and by trans mit ting nu mer ous se ri ous dis eases (eg,
[1]
Cra nial morphometric and fine scale ge netic vari abil ity of two ad ja cent Mastomys natalensis (Rodentia: Muridae) pop u la tions
Aude LALIS, Michel BAYLAC, Jean François COSSON, Rhodes H. MAKUNDI, Rob ert S. MACHANG’U and Christiane DENYS
Lalis A., Baylac M., Cosson J. F., Makundi R. H., Machang’u R. S. and Denys C. 2009. Cra nial morphometric and fine scale ge netic vari abil ity of two ad ja cent Mastomys natalensis (Rodentia: Muridae) pop u la tions. Acta Theriologica 54: 000–000.
T h e o b je c ti v e o f t h i s m u l t i di s c i p l i na r y p r o je c t wa s t o st u d y t he intra-spe cific morphometric and ge netic vari abil ity be tween two ad ja cent pop u la tions of Mastomys natalensis Smith, 1834 liv ing in dif fer ent en vi ron ments. The study of mi cro-evo lu tion ary pro cesses at work by us ing geo met ri cal morphometrics al lowed us to de fine two groups, char ac ter ized by dif fer ent fea tures of the skull shape. Us ing mo lec u lar microsatellites anal y sis, we showed that the two pop u la tions ex changed high gene flow and could be con sid ered as a sin gle panmictic unit. These re sults sug gest that this widely-dis trib uted spe cies ex hib its a lo cal pop u la tion-level dif fer en ti a tion in shape vari a tion of skulls, prob a bly due to dif fer ent eco log i cal sit u a tions. We spec u late that the vari abil ity in the cra nial char ac ter is tics (con nected with the feed ing abil ity) could re veal a lo cal ad ap ta tion pref er en tially based on the food avail abil ity. We pro pose an ex pla na tion link ing the shape dif fer ences to the fit ness gain in the ex ploi ta tion of re sources avail able in the two en vi ron ments. Since we sug gest a po ten tial dif fer en ti a tion pro cess be tween pop u la tions, we be lieve that the two groups con sti tute even better mod els to un der stand the fac tors in volved in the early stages of lo cal ad ap ta tions. The aim of such stud ies is to pro vide a better un der stand ing of the eco nom i cal im por tance of this spe cies and its re mark able ca pa bil ity to pro lif er ate.
UMR CNRS 5202 – USM 601, Origine, Struc ture et Evo lu tion de la Biodiversité, Département Systématique et Evo lu tion, Muséum Na tional d’Histoire Naturelle, CP 51, 55 rue Buffon, 75005 Paris, France, e-mail: [email protected] (AL, MB, CD); Plate-forme Morphométrie MNHN-CNRS IFR 10145, rue Buffon F-75005 Paris, France (AL, MB); Cen tre de Biologie et Gestion des Pop u la tions (UMR22), INRA, Cam pus In ter na tional de Baillarguet, CS30016, 34988 Montferrier sur Lez cedex, France (JFC); Ro dent Re search Pro ject, Sokoine Uni ver sity of Ag ri cul ture, Morogoro, Tan za nia (RHM, RSM)
Key words: Mastomys natalensis, intra-spe cific vari abil ity, skull shape vari a tion, geo met ric morphometrics, ge netic dif fer en ti a tion, microsatellites
the bac te ria Yersina pestis, the etiologic agent of plague and Lassa fe ver vi rus) (Monath et al.
1974, Kilonzo et al. 1992, Gratz 1997, Mwanjabe and Leirs 1997, Demby et al. 2001, Mwanjabe et al. 2002). Mastomys natalensis Smith, 1834 is a com mon anthropophilous spe cies and has the larg est geo graphic dis tri bu tion within the ge nus (Granjon et al. 1997). It shows all the char ac ter - is tics of a pest spe cies: high breed ing rate, wide - spread out breaks, strong ca pac ity of dis per sion and col o ni za tion (Leirs 1995). In Tan za nia, some pop u la tions of M. natalensis un dergo ir reg u lar pop u la tion ex plo sions with den si ties as high as 1400 rats per hect are (Mwanjabe et al. 2002).
Pest con trol on fields be comes a ne ces sity be - cause the ma jor ity of farm ers are smallholders (Mwanjabe and Leirs 1997). The eco log i cal op - por tun ism of M. natalensis and its ten dency to be dom i nant in the ro dent com mu ni ties de fine this spe cies as a good model in the study and un - der stand ing of the dis tri bu tion mech a nisms by pop u la tion ge net ics. Only one pop u la tion ge - netic anal y sis on M. natalensis on a small geo - graphic scale us ing microsatellites has been re al ized in the past (P. van Hooft and J. F.
Cosson, pers. comm.). Al though of great in ter est in the frame of pest con trol and hu man dis ease stud ies, gene flow rates within and be tween pop - u la tions of M. natalensis are not well-known.
Vari a tion in phenotypic and ge netic traits across geo graphic space is an ubiq ui tous phe - n o m e n o n e x h i b i t e d b y m o s t o r g a n i s m s (Monteiro et al. 2003) and the re la tion ship be - tween ge no type and phe no type is a re cur rent ques tion in mod ern bi ol ogy. The de scrip tions of mor pho log i cal ver sus ge netic char ac ters in the pat terns of vari a tions within and be tween pop u - la tions are fun da men tal for de fin ing bound aries of in de pend ent evo lu tion ary units in na ture (Dos Reis et al. 2002a). The study of phenotypic dif fer en ti a tion is rel e vant, not only as an ap - prox i ma tion to the un der ly ing ge netic dif fer en ti - a tion, but also be cause re al ized phe no types r e s u l t f r o m t h e i n t e r f a c i n g b e t w e e n morphogenetic rules, eco log i cal con di tions and de ter min is tic and sto chas tic evo lu tion ary forces (Monteiro et al. 2003). For the phe no type, the ma te rial ba sis of the dif fer ences is de fined in terms of size and shape (Monteiro et al. 2003).
With dif fer ent tech niques of geo met ri cal morphometrics which al low a quan ti fi ca tion of shape vari a tions, some ap proaches have been pos si ble in the dis crim i na tion and clas si fi ca tion of spe cies (Dobigny et al. 2002, Cordeiro et al., in press) and in anal y sis of shape evo lu tion (Auffray et al. 1996, Mullin et al. 2004, Tay lor et al. 2005). The geo met ri cal morphometric ap - proach (Bookstein 1991, 1996, Rohlf 2000) has al ready been suc cess fully ap plied to in ves ti gate as pects of mor pho log i cal vari a tion in the e c h i m y i d r o d e n t , T r i c h o m y s a p e r e o i d e s (Monteiro et al. 1999, Duarte et al. 2000, Dos Reis et al. 2002a, b), in Mus musculus (Auffray et al. 1996, Alibert et al. 1997, Debat et al. 2000), in Dasymys (Mullin et al. 2004) and in Otomys saundersiae (Tay lor et al. 2005).
The ob jec tives of this study are to elu ci date qual i ta tively and quan ti ta tively intra-spe cific mor pho log i cal and genetic vari abil ity at a populational scale. Pat terns of lo cal vari a tion in cra nial shape among pop u la tions of the ro dent spe cies M. natalensis in re la tion to ge netic dif - fer en ti a tion are in ves ti gated us ing the frame - w o r k o f g e o m e t ri c a l mo r p h o me t r i cs a nd microsatellites analysis.
Ma te ri al and methods
Sam ples
Spec i mens came from the ro dents col lec tion of Mu seum Na tional d’Histoire Naturelle de Paris. All in di vid u als were col lected in July 2000 at the edge of the Selous Game Re - serve (Tan za nia). Cytogenetic anal y ses were con ducted in the field and in lab o ra tory for ac cu rate iden ti fi ca tion of spe - cies M. natalensis (Denys et al., in press). Two dif fer ent hab i tats have been sam pled with the same trap ping ef fort (100 Sherman traps per lo cal ity for three con sec u tive nights). The sam pling ef fort yielded 137 Mastomys in di vid u - als (71 males, 66 fe males). These in di vid u als came from two dis tinct pop u la tions sep a rated by 5 km. The first pop u la tion (107 in di vid u als) lived in an ag ri cul tural area (dry maize field) close to a vil lage, a typ i cal hab i tat for the spe cies in Tan za nia. The sec ond pop u la tion (30 in di vid u als) was lo - cated in a swampy area with a per ma nent water hole in a
“de graded miombo” (sa vanna of fruit trees), a less typ i cal hab i tat for the spe cies.
Age de ter mi na tion
There are nu mer ous meth ods for age es ti ma tion, both rel a tive and ab so lute (re viewed by Mor ris 1972, Pucek and Lowe 1975, Poulet 1980). Among them, the ma jor ity of age cri te ria based on ex ter nal (weight, body length) or in ter nal mor pho log i cal (bone de vel op ment, tooth wear, eye lens weight) char ac ters have been proven to be im pre cise (Le Louarn 1971, Lalis et al. 2006). Al though body weight is not al ways di rectly and lin early re lated to age in Mastomys, it can be used as an ef fec tive age es ti ma tor (Leirs 1995). It was thus pos si ble to group the an i mals in three “re pro duc - tive age groups” ac cord ing to their weight (Leirs 1995): “Ju - ve niles” (an i mals be low 25 grams; they rep re sent the very young in di vid u als), “Sub adults” (sex u ally ma ture, weight - ing be tween 25 and 40 grams) and “Adults” (above 40 grams).
Geo met ric morphometric anal y sis
Skull land marks
Our sam pling in cluded 129 in di vid u als for the dor sal side of the cra nium and 104 in di vid u als for the ven tral side, and ex cluded all spec i mens with miss ing land marks.
Three-di men sional land marks, as sumed to be ho mol o gous from in di vid ual to in di vid ual, were iden ti fied for the dor sal and ven tral sides of the skull. They in cluded points of su - ture be tween bones or points of in ter sec tion be tween su - tures (25 land marks for dor sal side and 35 land marks for ven tral side; Fig. 1). Each in di vid ual skull was dig i ta lized for each side with the Re flex Mi cro scope (re flex Mea sure - ments Ltd.).
Nasal
Anterior palatine foramen Palatine process
1st molar 2nd molar 3rd molar Palatine
External pterygoid process Internal pterygoid process Basisphenoid canal Basisphenoid
Occipital
Occipital condyle Incisor
Premaxilla
Maxilla
Zygomatic Presphenoid
Squamosal Cranio-pharyngeal canal
Tympanic bulla External auditory meatus
Hypoglossal canal Foramen magnum Occipital condyle
Occipital Squamosal
Tympanic bulla Lacrymal
Zygomatic Nasal
Premaxilla
Maxilla
Frontal
Parietal
Interparietal
(a) (b)
2 1 3
4 5
8
14 1618
10 11 9
18 15 6 7 12 35
22 34
26 27
24 31
30 29 28
21 23
20 32
33
25
20 22 21
18
23 14
16 24
12
11 6 8 4
10 25 1 2 3
9 7 5
13 15
17 19
19 17
Fig 1. Land mark lo ca tion for a) the dor sal and b) the ven tral views of the skull of Mastomys natalensis (Cook 1965). Dor sal view: tip of the nasals (1), most an te rior points at na sal - premaxillary su ture (2, 3), an te rior pro jec tion of zygomatic (4, 5), in - ter sec tion of premaxilla and fron tal (6, 7), na sal – fron tal su ture (8, 9, 25), su ture of premaxilla and maxilla over lach ry mal cap sule (10, 11), fron tal – pa ri etal su ture (12, 13, 24), most me dial point at interorbital con stric tion (14, 15), back of zygomatic notch (16, 17), in ter sec tion of pa ri etal – interparietal and supraoccipital su tures 18, 19), back of the lateroccipital pro tu ber - ances (20, 21, 22), mid point of pa ri etal interparietal su ture (23). Ven tral view: tip of the nasals (1), antero-lat eral ex trem ity of left or right in ci sive alveolus (2, 3), an te rior pal a tine fo ra men (4, 5), pos te rior pal a tine fo ra men (6, 7), back of zygomatic plate (8, 9), in ter sec tion be tween left or right an te rior end of mo lar and man di ble (10, 11), in ter sec tion be tween left or right pos te - rior end of mo lar and man di ble (12, 13), su ture be tween jugal and squamosal in the left or right zygomatic arch (14, 15), front of glenoid fossa on squamosal root of zygomatic arch (16, 17), su ture be tween left or right pa ri etal and squamosal (18, 19), back of ex ter nal open ing of au di tory bullae (20, 21), lat eral points of sphenoccipital su ture (22, 23), styloinastoid fo ram ina at the pos te rior bor der of ex ter nal au di tory meatus (24, 25), an te rior ex trem ity of oc cip i tal (26, 27), pos te rior in ter sec tion be - tween fo ra men mag num and oc cip i tal condyle (28, 29), pos te rior ex trem ity of fo ra men mag num (30), an te rior ex trem ity of fo - ra men mag num (31), an te rior ex trem ity of oc cip i tal (32), cranio – pha ryn geal ca nal (33), con tact point be tween pal a tine and presphenoid (34), con tact point be tween maxilla and pal a tine (35).
Morphometric anal y sis
Ven tral and dor sal land marks con fig u ra tions were su - per im posed sep a rately by a least-squares gen er al ized Pro - c r u s t e s a l g o r i t h m . T h e g e n e r a l i z e d P r o c r u s t e s su per im po si tion pro ce dure scales, trans lates and ro tates the con fig u ra tions to min i mize the summed squared dis - tances be tween cor re spond ing land marks (Rohlf and Slice, 1990). The size mea sure ment was cal cu lated by the log of cen troid size, de fined as the square root of the summed squared dis tances from each land mark to the over all cen - troid. Land mark con fig u ra tions were used to com pute con - sen sus con fig u ra tion for dor sal and ven tral sides of the skull for pop u la tion sam ples of Mastomys natalensis as the grand mean of aligned spec i mens from all pop u la tions. Re - sid u als from the mean land mark lo ca tions (= Pro crus tes re - sid u als) to gether with cen troid size con sti tute the shape used in the sta tis ti cal anal y ses.
Multivariate sta tis ti cal anal y ses
The size pat terns of vari abil ity were ana lysed by anal y - sis of vari ance (ANOVA) re lated to age, sex and lo cal ity (and the in ter ac tions). Re gres sion pa ram e ters al low pre dic - tion and vi su al iza tion of the shape changes in re la tion to the spec i fied vari able. We used this ap proach to vi su al ize the shape vari a tion along the ca non i cal axes, as well as for the allometrical pat terns mod el ing (Flury 1984, Airoldi and Flury 1988) in or der to de scribe the allometry-free shape dif fer ences among pop u la tions. These allometry-free pat - terns were sub mit ted to a discriminant anal y sis us ing multivariate anal y sis of covariance (MANCOVA) with log10 cen troid size as a covariate. The dor sal and ven tral views have been com bined on a sin gle dataset for the sta tis ti cal anal y ses (Ca non i cal Variate Anal y ses) by us ing the R lan - guage (R.2.0.1, 2004). Su per im po si tions and vi su al iza tions were cal cu lated with the ‘Rmorph’ li brary for R (Baylac 2007). Due to the small sam ples sizes, shape anal y ses were re al ized us ing the 10 first prin ci pal com po nents (PCs) ex - tracted from Pro crus tes re sid u als which ex plained 73.8% of the to tal vari ance for the ven tral side and 79.4% for the dor - sal side. This num ber of PC was se lected to min i mize the cross-val i dated misclassification per cent ages (Baylac and Friess 2005).
Ge netic mark ers anal y sis
Sam ple col lec tion
We used 31 sam ples of M. natalensis which came from the two stud ied pop u la tions (23 for pop u la tion 1 and 8 for pop u la tion 2). To tal DNA was ex tracted from liver and spleen pre served in 70% eth a nol us ing the Dnease Tis sue Kit (Qiagen).
Microsatellite anal y sis
For geno typ ing, we used a set of 10 microsatellite mark - ers with dinucleotide re peats and spe cially de vel oped for Mastomys spe cies (Galan et al. 2004). The microsatellites were am pli fied in a sin gle mul ti plex PCR re ac tion with the Qiagen Mul ti plex PCR kit fol low ing man u fac turer’s in struc -
tions. The PCR prod ucts were tested on monocapillary ABI PRISM 310 Ge netic Analyser and GENESCAN anal y sis soft ware (Ap plied Biosystems).
Sta tis ti cal anal y sis of microsatellite data
Mean num ber of al leles per lo cus, ob served (Ho) and ex - pected (He) heterozygosity (Nei 1987) were cal cu lated over all loci at each sam pling lo ca tion us ing the soft ware GENETIX v4.04 (Belkhir 2001). Tests for de par tures from Hardy-Wein berg equi lib rium, for genotypic link age dis equi - lib rium and in de pend ence be tween pop u la tions, and al lele fre quency were per formed for all pop u la tions us ing the prob a bil ity tests in GENEPOP v3.3 soft ware (Ray mond and Rousset 1995). Markov chain method was used to pro vide an un bi ased es ti ma tion of the ex act p-value (more than 5 al - leles) and to test genotypical di ver gence among pop u la tions for all loci and pop u la tion pairs. F-sta tis tics: FST (pop u la - tion dif fer e n ti a t ion) and FI S ( h e te r o z yg o t e de f i ci t , Watterson 1978) were cal cu lated ac cord ing to Weir and Cockerham (1984) us ing GENEPOP soft ware, and the sig - nif i cance of Fst es ti mates was de ter mined us ing a per mu ta - tion test im ple mented in the GENETIX v4.04 pack age (1000 per mu ta tions, one-sided test of the null hy poth e sis of no ge netic dif fer ence among the groups of pop u la tions).
Re sults
Age de ter mi na tion: “Body weight” method The in di vid u als were grouped into three age classes de pend ing on their body weight. The age class dis tri bu tion of the two pop u la tions showed that the ma jor ity of in di vid u als for the two samplings be longs to the ju ve nile class (93/137 in di vid u als). A Fisher ex act test of sig nif i cance showed that the two pop u la tions were not significatively dif fer ent in terms of age (p = 0.1079) and sex (p = 0.1138).
Geo met rical morphometrics
Size vari a tion
Nei ther ‘pop u la tion’ (ANOVA: F = 0.2188, p = 0.7896) nor ‘sex’ (F = 2.3874, p = 0.0657) pa ram e - ters were sig nif i cant on size dif fer ences (Ta ble 1). As ex pected, cen troid size was strongly cor re - lated to age (F = 78.82, p < 0.001).
Shape vari a tion
The re sults of multivariate anal y sis of covariance (Ta ble 1) on geo met ri cal shape with
“pop u la tion”, “sex”, “age” and “cen troid size” as in de pend ent vari ables, and their in ter ac tion re - vealed two dis tinct pat terns in shape vari a tion.
The “pop u la tion” and “age” fac tors were highly sig nif i cant (MANCOVA: F = 3.1285, p < 0.001; F
= 6.4956, p < 0.001 re spec tively). The re sults for the “pop u la tion” pa ram e ter pre sented low misclassification per cent ages (dor sal: 25.20%, ven tral: 18.75%) (Ta ble 2). The low over all misclassification rates were mainly due to the larger sam ple size in pop u la tion 1. The “sex” pa - ram e ter was not sig nif i cant (MANCOVA: F = 1.0155, p = 1.0025) (Ta ble 1). For the ‘pop u la - tion’ fac tor, shape trans for ma tions along the discriminant axe 1 (PC1) ac count ing for 32%
(ven tral view) and 27% (dor sal view) of the vari - a tion in skull shape, sum ma rized the vari a tion in the antero-pos te rior ori en ta tion. PC2 ac - counted for 11.6% (ven tral view) and 8.5% (dor - sal view) of the vari ance which il lus trated with lat eral de for ma tion at the level of the tym panic bulla. Fig ure 2 showed the vi su al iza tions of Pro - crus tes su per im po si tions for the land marks ac - cord ing the “pop u la tion” (mean shape of pop u la tion 1 ver sus mean shape of pop u la tion 2).
In ven tral view, the pop u la tion 1 seemed to have a skull rel a tively shorter and wider at the level of the tym panic bulla. More over, the pos te rior part of cra nial shape ap peared to be rel a tively dif fer ent be tween both pop u la tions: the or bital limit of su ture be tween pa ri etal and squamosal and the ex trem i ties of tym panic bullas seemed to be or ga nized dif fer ently in term of space dis - po si tion (Fig. 2). We also ob served that the lengths of mo lar rows were dif fer ent be tween the both pop u la tions since it is larger in pop u la - tion 1 than in pop u la tion 2 (Fig. 2). In dor sal view, pop u la tion 2 seemed to pres ent a lon ger and thin ner shape than in the pop u la tion 1 with a more pro nounced ros trum (con tact points be - tween na sal, premaxillary and fron tal which were sit u ated in the back of the skull) (Fig. 2). In con clu sion and ac cord ing to the re sults ob tained on the 2 sides, a sig nif i cant dif fer ence in the skull shape be tween pop u la tions was ob served:
a shorter cra nial shape for pop u la tion 1 and an im por tant dif fer ence in the lengths of mo lar rows be tween the pop u la tions (Fig. 2).
Ta ble 1. ANOVA on cen troid size and MANCOVA on Pro crus tes re sid u als from two pop u la tions of Mastomys natalensis. Ven tral and dor sal views were pooled for anal y ses. Log trans formed cen troid size was used as covariate to ac count for allometric ef fects of size vari a tion. Sig nif i cance codes: **** – 0, *** – 0.001, ** – 0.01, * – 0.05.
ANOVA – cen troid size 2 Views (dor sal + ven tral) MANCOVA - skull shape
Vari ables df F-val ues P-val ues Covariates df F-val ues P-val ues
Pop 1 0.2188 0.7896 Pop 1 3.1285 0.0009***
Sex 1 2.3874 0.0657. Sex 1 1.0155 1.0025
Age 2 78.8224 1.588e-15*** Age 2 6.4956 1.478e-11***
In ter ac tion Pop.Sex 1 0.1458 0.8763 Log cen troid size 1 0.4159 0.9563
In ter ac tion Sex.Age 2 1.4687 1.5334 In ter ac tion Pop.Sex 1 0.7466 0.6148 In ter ac tion Pop.Age 2 0.0878 0.8756 In ter ac tion Sex.Age 2 1.7014 0.5499 In ter ac tion Pop.Age 2 1.2520 0.1856
Ta ble 2. Re-clas si fi ca tion per cent ages of in di vid u als of Mastomys natalensis to two pop u la tions (Pop1 and Pop2) ac cord ing to discriminant anal y ses of skull shape us ing cross-val i da tion.
Vari able : pop u la tion
From/To Pop1 Pop2 Num bers
Ven tral side
Pop1 84 5 89
Pop2 16 7 23
To tal of missclassified in di vid u als (%) = 18.75 Dor sal side
Pop1 92 8 100
Pop2 24 3 27
To tal of missclassified in di vid u als (%) = 25.20
Microsatellite study
Num ber of al leles and allelic di ver sity
All microsatellite loci showed a high ge netic di ver sity: the num ber of al leles per lo cus ranged from 9 to 24 with a mean num ber of al leles per lo cus = 15.3. The mean num ber of al leles per lo - cus and per pop u la tion was of 10.35 (Ta ble 3).
No link age dis equi lib rium be tween loci oc curred for all mixed pop u la tions ex cept be tween the two loci which dis played a low dis equi lib rium (0.0441 < 0.05). How ever, in de pend ence among loci was as sumed in the sub se quent anal y ses.
Intrapopulation ge netic di ver sity
The num ber of al leles per lo cus ranged from 7 to 16 for pop u la tion 1 and from 4 to 12 for pop u - la tion 2 with a mean num ber of al leles per lo cus re spec tively of 11.8 and 8.9 (Ta ble 3). The ex - pected heterozygosity was rel a tively high. Ac - cord ing to lo cus and pop u la tion (mean num ber = 0.889), it var ied be tween 0.884 and 0.894 (Ta ble 3). Over all, the av er age val ues for ex pected heterozygosity were sim i lar be tween lo cal i ties.
All loci for each pop u la tion were at Hardy Wein - berg equi lib rium (HWE).
Populational dif fer en ti a tion
F-sta tis tics re vealed a non-sig nif i cant genotypical dif fer en ti a tion among pop u la tions.
The FST value was very low (0.00150). The FIS value un der lined the ab sence of Wahlund ef fect (a sub di vided pop u la tion con tains fewer het ero - zy gotes than pre dicted de spite the fact that all sub di vi sions are in Hardy-Wein berg equi lib - rium). We ap plied the FIS test to mixed pop u la - tions 1 and 2 (= hy poth e sis of a unique (a) (b)
Dorsal view
Back view
Lateral view
Fig. 2. Shape trans for ma tions of the skull along the discriminant axes for the two pop u la tions of Mastomys natalensis. Solid lines de pict group 1; dot ted lines de pict group 2. Dor sal (a) and ven tral (b) sides.
Ta ble 3. Ge netic di ver sity at sam pling sites.
Pop u la tion Mean no.
al lele/lo cus
Heterozygosity Ex pected (He) Ob served (Ho)
1-Vil lage 11.8 0.884 0.895
2-Swamp 8.9 0.894 0.900
Mean 10.35 0.889 0.898
pop u la tion) in or der to test the heterozygote de - fi ciency. We no ticed a heterozygote de fi ciency rel a tively sim i lar or in fe rior to those ob served in the mean value for iso lated pop u la tions (FIS Pop1 = –0.017 and FIS Pop2 = –0.0025). The hy - poth e sis of a panmictic pop u la tion be tween the pop u la tion 1 and 2 was con firmed.
Dis cus sion
We have shown that the two pop u la tions were not significatively dif fer ent in terms of com po si tion in “age” and “sex”. The data rep re - sented in stan ta neous sam ples of the nat u ral pop u la tion in a dry pe riod and a drought zone.
We knew that breed ing was highly sea sonal for M. natalensis. It usu ally starts af ter the rain sea son in Tan za nia (in April) and lasts un til the dry sea son (Sep tem ber) (Stenseth et al. 2001), with some births ob served from May to Oc to ber (Lima et al. 2003). Adults die af ter the breed ing sea son and ju ve niles usu ally do not be come sex - u ally ma ture un til the next dry sea son (Lima et al. 2003). The age com po si tion of our samplings re flected the bi o log i cal cy cle of the spe cies in Tan za nia. In deed, the sam pling cam paign took place at the be gin ning of July, while the ju ve - niles of the year were pres ent in large num bers.
Dur ing the last pe riod (July, Au gust, and Sep - tem ber) adults be gan to die pro gres sively (Leirs et al. 1990), which ex plained their small rep re - sen ta tion in July. The cam paign, made in July 2003, took place dur ing a dry pe riod (Lalis et al.
2006). The ab sence of rain fall un til July was prob a bly re spon si ble for an im por tant mor tal ity in all age classes and an im por tant de lay of the de vel op ment of sur viv ing ju ve niles, which ex - plained the large num bers of sex u ally im ma ture ju ve niles. The ob ser va tion of no ges tat ing fe - males dur ing the sam pling con firmed the ex cep - tional con text of the win ter 2003, which was char ac ter ized by al most no breed ing.
The ge netic di ver sity in dex is co her ent be - cause He (ex pected heterozygosity) val ues higher than 0.82 were not spe cial for ro dents as it had al ready been ob served in Apodemus flavicollis (Gockel et al. 1997), Rattus fuscipes (Hinten et al. 1999), Peromyscus leucopus (Moss -
man and Waser 2001) and Mus musculus (Yu and Peng 2002). Such an im por tant ge netic di - ver sity seems to be re lated to large den sity pop - u la tions. In the pres ent study, ge netic re sults showed that the two pop u la tions were not two iso lated pop u la tions for breed ing. FST value ap - peared low and not - sig nif i cant. The ge netic dif - fer en ti a tion be tween pop u la tions was not sup ported by dif fer ences in allelic fre quen cies or sig nif i cant de vi a tions from HWE. There fore, no clear ge netic dif fer en ti a tion be tween ge netic pools was ob served. With the microsatellite study, it ap peared that the two pop u la tions ex - changed high gene flow and could be con sid ered as a sin gle panmictic unit. The pa ram e ters of ge - netic di ver sity were prob a bly re duced by the mi - gra tion phe nom e non. In deed, P. van Hooft and J. F. Cosson (pers. comm.) have un der lined the ex is tence of an im por tant mi gra tion be tween M.
natalensis pop u la tions in Tan za nia. They have showed that ju ve niles and adults were re spon si - ble for high mi gra tion rates. Leirs (1995) has dem on strated that the spe cies, used to short daily dis tances (10–15 m), was some times able to cover a dis tance of l00 m per day. Since our sam ples were mainly con sti tuted by ju ve nile in - di vid u als (1 to 3 months), we were in a con text where the mi gra tion phe nom e non could be im - por tant and could have led to a strong lo cal He and a low ge netic dif fer en ti a tion be tween field plots, which was in agree ment with the ob served re sults.
In geo met ri cal morphometrics, the sta tis ti cal re sults dem on strated that the “sex” vari able was not sig nif i cant and con firmed the ab sence of a sex ual di mor phism for our sam ples. In murids, sex ual di mor phism has rarely been dem on - strated on the ba sis of clas si cal morphometrics dis tance anal y sis. For M. natalensis of South Af - rica, Smithers et al. (1979) showed that males were larger than fe males by body mea sure ments but the sta tis ti cal re sults were not sig nif i cant.
Delany (1975) raised an op po site ten dency for a Ugan dan pop u la tion but ob tained a sta tis ti cally non sig nif i cant re sult (fe males were larger than males), a point also dem on strated in pop u la tions in Sen e gal by Duplantier (1988). Size and shape change across ages. The “age” pa ram e ter was very sig nif i cant in the both views (for Cen troid
size and Pro crus tes re sid u als) and prob a bly cor - re sponded to growth allometry. The sta tis ti cal re sults con firmed that the “pop u la tion” vari able was sig nif i cant and un der lined a sig nif i cant dif - fer ence in the skull struc ture be tween pop u la - tions. Two sides gave some dis tinct per spec tives on the vari a tion of cra nial shape and on the or - ga ni za tion of mor pho log i cal vari a tion be tween stud ied pop u la tions. On the vi su al iza tions, cra - nial shape of pop u la tion 1 was ob served to be shorter but higher than the cra nial shape of pop - u la tion 2. With the ‘allometry-free’ model, the hy poth e sis of allometric dif fer ences be tween pop u la tions is di rectly tested and can not ex plain t h e m o r p h o l o g i c a l d i f f e r e n c e s b e t w e e n populations.
Such shape vari a tion con firms the strong intra spe cific vari abil ity in M. natalensis. Some dif fer ences be tween pop u la tions were sit u ated in ar tic u lar zones, es pe cially on the ven tral side.
These ob ser va tions could be re lated to a dif fer - ence in a diet and con se quently in a mor pho log i - cal ad ap ta tion. In deed, some mul ti ple ge netic and en vi ron men tal fac tors in flu ence the cra nial shape but sev eral lines of ev i dence sug gest that changes in diet and food pro cess ing tech nol ogy con trib ute to some pro por tion of vari a tions in fa - cial size and shape (Lieberman et al. 2004). Most ex per i men tal data on fa cial growth re sponses to masticatory load ing come es pe cially from pri - mate stud ies (Beecher et al. 1983, Bouvier and Hylander 1996). How ever, some ex per i ments on rats re veal that the in di vid u als raised on soft food have smaller jaw adductor mus cles, gen er - ate lower man dib u lar strains, and have sig nif i - cantly de creased an te rior fa cial height, shorter man di bles, and smaller mus cle at tach ment ar - eas (Engström et al. 1986, Kiliaridis et al. 1986, Yamada and Kimmel 1991). In this study, the in di vid u als of pop u la tion 1 (vil lage) feeds mainly on some maize, rice and seeds, which are harder than green veg e ta tion and ar thro pods that were con sumed by pop u la tion 2 “swamp”.
This could ex plain the mor pho log i cal dif fer ences but. Thus, strain and nu tri tion both in flu ence jaw size, but only strain af fects jaw shape. The changes in tym panic bulla un der line a light asym me try be tween pop u la tions. It would be in - ter est ing in our case to re al ize a de tailed study
of fluc tu at ing asym me try, which is gen er ally con sid ered as the only form of asym me try that can serve as a use ful in di ca tor of or gan isms sub - jected to stress (Palmer and Strobeck 1986, Leary and Allendorf 1989, Marchand 2003) in or der to know if the ‘vil lage’ pop u la tion is in a more stress ful sit u a tion than the ‘swamp’ pop u - la tion.
I n s u m m a r y , w i t h t h e g e o m e t r i c a l morphometric study, two groups of pop u la tions liv ing in dif fer ent en vi ron ments have been shown to be sig nif i cantly dif fer ent, based on some dis tinct mor pho log i cal fea tures. We could raise the hy poth e sis of a lo cal ad ap ta tion and mor pho log i cal dif fer en ti a tion with out ge netic struc tur ing. We knew that dif fer ent an a tomic forms could par ti tion their food re sources by de - vel op ing lo cal ad ap ta tions (vari a tions in feed ing hab its and spa tial dis tri bu tion) for the dif fer ent hab i tats they used (Dynes et al. 1999). In deed, Patton and Brylski (1987) have shown clearly body size vari a tion in pocket go phers in hab it ing al falfa crops ver sus nat u ral dry en vi ron ments, Tay lor et al. (2005) have un der lined sig nif i cant geo graph ical dif fer ences in cra nial shape be - tween east ern and west ern Cape pop u la tions of Otomys saundersiae, oc cu py ing dis tinct eco log i - cal biomes. Sev eral stud ies have shown the pres - ence of lo cal pop u la tion-level dif fer en ti a tion in size and shape vari a tion of mo lars, skulls and man di bles in mam mals (Cardini et al. 2003, Cardini and O’Higgins 2004, Polly 2007) More - over, the sig nif i cant skull shape vari a tion be - t w e e n p o p u l a t i o n s s h o w e d t h a t t h e mor pho log i cal pat terns evolved in Arvicanthis from Su dan are dis tinct enough to al low iden ti fi - ca tion of pop u la tions (Rahman Abdel 2005). We also know al ready that Af ri can murids are able to vary their diet be tween sea sons which sug - gests also a re mark able power of ad ap ta tion to lo cal con di tions in that group (Gliwicz 1987). In this study, some in di vid u als would be more anthropophilic (us ing clas si cal hab i tat) whereas oth ers, pushed by the harsh en vi ron men tal con - di tions of a drought (no breed ing and intra-spe - cific com pe ti tion for food), were able to join a new hab i tat (swamp type) like a more hu mid area that served as a ref uge and would have de - vel oped the first signs of a lo cal ad ap ta tion. The
dif fer ences of cra nial shape could per haps re - flect the lo cal ad ap ta tion with out ge netic struc - tur ing, ie mor pho log i cal dif fer en ti a tion in the pres ence of gene flow, as a re sponse to re solve prob lems of food avail abil ity (Her ring 1993, Wood and Lieberman 2001, Debat and Da vid 2001, Kingsolver et al. 2002, Piersma and Drent 2003, Lieberman et al. 2004). Be cause we had no anal y sis of stom ach con tents re ported to ver ify the diet of each pop u la tion, the ex pla na tion link - ing the shape dif fer ences to the fit ness gain in the ex ploi ta tion of re sources avail able in the two en vi ron ments is rea son able but spec u la tive.
In sum mary, we pro pose that the widely-dis - trib uted Mastomys spe cies ex hib ited a lo cal mor - pho log i cal dif fer en ti a tion, prob a bly due to dif fer ent eco log i cal sit u a tions. We spec u late that the vari abil ity in the cra nial char ac ters (con nected with the abil ity of feed ing) could re - veal pref er en tially a rapid and lo cal ad ap ta tion based on the food avail abil ity. Since we sug - gested a po ten tial dif fer en ti a tion pro cess be - tween pop u la tions, the two groups con sti tuted even better mod els to un der stand the fac tors in - volved in the early stages of lo cal ad ap ta tions. It would be very in ter est ing to study these two pop u la tions in more de tail to in ves ti gate the intra and inter-pop u la tion mor pho log i cal vari - abil ity on a lo cal scale and diet. The aim of such stud ies would be to pro vide a better un der stand - ing of this eco nom i cally im por tant spe cies and its re mark able ca pa bil ity to pro lif er ate.
Ac knowl edge ments: We would like to thank Mr Ngouli, Mr White (COSTECH), Mr Se vere (Wild life Di vi sion Tan za nia) and the field staff for their as sis tance in the “field” re al iza - t i o n o f t h i s p r o j e c t ( R e s e a r c h P e r m i t N o . 2003-152-CC-2003-83, Tan za nia com mis sion for Sci ence and Tech nol ogy, COSTECH). We are grate ful to E.
Lecompte, A. Delapré, M. Galan, C. Houssin, V. Aniskine, and As so ci a tion ‘Adrar des Iforas’ (R. Cornette and S. Mou - lin) for the treat ment and anal y sis of spec i mens. We thank F. Guy for his sug ges tions. We also ac knowl edge the re view - ers for their at ten tive read ing of the manu script.
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