T h e s u b s p e c if ic t a x o n o m y o f T r y p a n o s o m a b r u c e i
J. R. BAKER*
S u m m a r y :
Trypanosoma brucei was first seen by David Bruce in 1894, in the blood of a cow in South Africa, and named in his honour in
1899. Trypanosomes seen in the blood of an Englishman in The Gambia in 1901 were named T. gambiense in 1902. Finally, in
1909, trypanosomes from the blood of an Englishman in Zambia ("Rhodesia") were named T. rhodesiense. Since then there has been continuous debate about the interrelationships of these three
"species". Studies of the molecular biology of these trypanosomes, mainly analyses of their isoenzymes and deoxyribonucleic acid, now appear to have shown that T. "rhodesiense" cannot be distin
guished from T. brucei brucei by any valid and consistent criterion, while T. "gambiense" probably does constitute a valid subspecies of T. brucei. There is still doubt whether populations of T. brucei are predominantly clonal or sexual. W hile some form of genetic exchange undoubtedly can occur in this species, its nature and fre
quency are unknown and there is evidence that the population structure of T. brucei is essentially clonal.
KEY WORDS :
Trypanosoma brucei.
history, subspecific taxonomy, population structure.
R ésum é
: Lat a x o n o m ies o u s-s p é c if iq u ed e Tr y p a n o s o m ab r u c e iTrypanosoma brucei a été observé pour la première fois en 1894 par David Bruce, dans le sang d'une vache d'Afrique du Sud, et ainsi nommé en son honneur en 1899.
Les trypanosomes observés dans le sang d'un anglais en Gambie en 1901 furent appelés T. gambiense en 1902. Enfin, en 1909, les trypanosomes provenant du sang d'un anglais en Zambie ("Rhodésie") furent appelés T. rhodesiense. Depuis, les interrelations entre ces trois "espèces" ont fait l'objet d'un débat continuel. L'étude de la biologie moléculaire de ces trypanosomes, principalement l'analyse de leurs isoenzymes et acide désoxyribonucléique, sem
blent avoir montré que T. “rhodesiense" ne peut être différencié de T.
brucei brucei par aucun critère valable et permanent, alors que T.
gambiense constitue vraisemblablement une sous-espèce de T. bru
cei. Il persiste un doute quant au caractère principalement clonal ou sexuel des populations de T. brucei. Bien qu'une mutation génétique puisse indubitablement survenir chez cette espèce, sa nature et sa fréquence sont inconnues et il est démontré que la structure démo
graphique de T. brucei est essentiellement clonale.
MOTS CLÉS :
Trypanosoma brucei.histoire, taxonomie sous-spécifique, structure démographique.
BACKGROUND
I
n Novem ber 1894, just over 100 years ago, David Bruce (Fig. 1) (then a captain in the British army and subsequently knighted as Sir David) and his wife Mary arrived in a small village ( ‘a m agistrate’s office and a few mud huts’ : Joubert et al., 1993) called U bom bo ( O b o n jen i in the local language, accor
ding to Jo u b e r t e t a l ., 1 9 9 3 ) in K w aZ u lu , N atal Province, South Africa. They had been sent there by the military authorities to try to discover the cause o f a disease know n as n a g a n a w hich was devastating the cattle o f the inhabitants o f the region and the horses used by the administrators o f what was then a British colony.
Bruce and his wife established their hom e (and labo
ratory) in a rectangular building (Fig. 2), built mainly o f m ud o n a fra m e w o rk o f w a ttle ( in te r la c e d branches o f small trees and shrubs) apparently with a
* Royal So ciety o f T ropical M edicine and H ygiene, M anson H ouse, 26, Portland P lace, London, W 1N 4EY , England.
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fou n d ation o f sto n es (Jo u b e rt et a l., 1993; C ook, 1994).
Bruce started his investigation by examining daily the blood o f a ‘brown cow ’ suffering from n a g a n a . He observed bacteria, which he at first thought w ere pro
bably the cause o f the disease, but on the sixth day h e r e c o r d e d in h is n o te s th e p r e s e n c e o f
‘H aem ato zo a’ (M acArthur, 1955). H e su bsequ ently adm itted (B ru c e , 1915) that he originally thought these ‘Haem atozoa’ w ere small filariae, but within a short time (probably before the end o f 1894) Bruce had concluded that the haem atozoa (Fig. 3) w ere the cause o f n a g a n a and also o f ‘tsetse fly disease’ o f humans, w hich until then had b een thought to be a separate entity (Bruce, 1895, 1896). A few years later th e ‘H a e m a to z o o n ’ w as n am ed b y P lim m er and Bradford (18 9 9 ) as T ry p a n o s o m a b r u c ii [sic], Bruce having sent an infected dog to England. The specific name, w hether printed thus due to a typesetter’s error or a lapse on the part o f the authors, was soon cor
rected to T. b r u c e i (Nabarro, 1907, p. 112 footnote 1, stated that the ch an g e w as m ade by Laveran and Mesnil, but cited no reference).
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3J. R. BAKER
Fig. 2. - D avid B r u c e ’s h u t in U b o m b o , 1 8 9 4 -1 8 9 5 (fro m B r u c e , 1915 : Lancet, ii, 1-6; p h o to g rap h kin dly su p p lie d b y D r G .C . C o o k [se e C ook, 19941).
Fig. 1. - D avid B ru ce (p h otog rap h taken probab ly in 1917, and kindly sup plied b y cou rtesy o f th e Administrator, Royal Society o f T rop ical M ed icine and H ygiene).
Fig. 3. - P hotograph o f D r J . E. Dutton (right) w ith, n ext to him , Mr H. Kelly, th e first kn ow n c a se o f hum an trypa
n o so m ia sis (1 9 0 1 ). O n e o f th e o th er tw o m en is Dr R. M. Forde; th e id en tity o f th e se c o n d is not kn ow n . T he o rig in a l p h o to g ra p h is a n n o ta te d in D utton ’s handw riting ‘T h e first c a se o f try p a n o so m ia s is’, w ith th e n a m e s o f K elly an d D utton. P h oto grap h kindly su p p lied b y co u rtesy o f Miss Patricia Miller, archivist, and th e Photograph ic D epartm ent o f th e Liverpool Sch o o l o f T ropical M edicine.
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P arasite, 1995, 2, 3-12 4Fig. 4. - Sk etch es o f ‘H aem atozo a’ in th e b lo o d o f a dog, from Plate V o f B ru ce (1 8 9 5 ). Later, colou red draw ings o f trypan osom es in B ru ce’s reports w ere m ade b y Mrs (later Lady) B ru ce; th ese sk etch es are cruder, and m ay have b e e n drawn by B ru ce him se lf (p h otog rap h kindly sup plied b y courtesy o f Ms M ary G ib so n , b ib lio g ra p h er, and th e V isual Aids D epartm ent o f th e London Sch ool o f H ygiene and T ropical M edicine).
Only two years after the naming o f T. bru cei, in May 1901, a doctor in the British colonial service in The Gambia named R. M. Fordei saw, in the blood o f a 42 years old Englishman ( ‘H. K.’), subsequently identified from the archives o f the Liverpool School o f Tropical Medicine as H. Kelly; see fig. 4), w ho was master o f a government steam boat on the Gambia river, what he d e s c rib e d as ‘sm all w o rm -lik e , e x tre m e ly a ctiv e bodies’ which he first thought w ere filariae. However, later in 1901 J. E. Dutton, o f the Liverpool School of T ro p ic a l M e d icin e , w as v isitin g B a th u rst in T h e Gambia. Forde show ed his patient, and the ‘worm s’ in his blood, to Dutton, w ho recognized them as trypa
nosom es (Forde, 1902). Dutton (1902) subsequently nam ed the parasite T. g a m b ie n s e . (D utton actually wrote ‘At present then it is im possible to decide defi
nitely as to the sp e cies, but if on further study it should be found to differ from the other disease pro
ducing trypanosomes I would suggest that it be called T ry p an osom a g a m b ie n s é (p. 467); although this is a
‘conditional proposal’ in the terms o f the International Code o f Zoological Nomenclature (third edition, 1985, article 15), such proposals made before 1961 do not prevent availability o f the nam e.)
On 17 N ovem ber 1909 trypanosom es w ere seen in the b loo d o f a 26 years old Englishm an w ho had been travelling along the valley o f the Luangwa river in Zambia (then north-east Rhodesia); he was thought to have b eco m e infected there in Septem ber 1909.
O n 2 D e c e m b e r h e w as a d m itte d to th e R o y al Southern hospital in Liverpool, under the care o f Dr
1. R obert M ichael Ford e w as b orn at C loyne, C ounty Cork, Ireland in 1862 and d ied ‘at his h om e in W orthing [Sussex, England] on March 27 [1942] at th e age o f 8 6 ’ (C.A.H ., 1948).
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Ronald Ross (w ho is now more famous for his colla
boration with Patrick Manson on the mosquito trans
mission o f malaria). The case history o f this patient was reported by Ross and Thom pson (1910, 1911); he w as su bjected to a horrifying battery o f attem pted therapies for his condition, including atoxyl, quinine, m ethylene blue, trypsin, amylopsin, succinam ide o f mercury, ‘izal o il’ [presumably a com m ercial house
hold disinfectant o f that name], trypan red, potassium iodide, ‘v a ccin e s’ prepared from his ow n trypano
som es grown in rats, and an extract o f rat peritoneal leucocytes. O n one occasion he was injected with 1 x 109 ‘dead’ trypanosom es in rat’s blood; this, the doc
tors reported, ‘seem ed to cause no harm’ (Ross and Thom pson, 1911). Perhaps not suprisingly, the unfor
tunate patient died on 29 Ju n e 1910. He was identi
fied by Ross and Thom pson (1910), and by Stephens and Fantham (1 9 1 0 ), o n ly as ‘W. A .’, a native o f Northum berland in the UK. D uggan (19 7 0 ) nam ed him as W. Armstrong, but gave the date o f his infec
tion as 1908; this seem s to be a mistake.
B efore his death, trypanosom es w ere isolated from W. A.’s blood and studied (in rats) by Drs Stephen and Fantham at the Liverpool S ch o o l o f T ro p ical M edicine. Stephens and Fantham (1 9 1 0 ) noted the virulence o f this strain, and reported the appearance o f posteronuclear trypomastigotes in the rats’ blood w hich (they thought) distinguished it from T. g a m b ie n s e ; they discussed the possibility that it was (i) a
‘variety’ or ‘local race’ o f T. g a m b ien se , (ii) a subspe
cies o f T. g a m b ie n s e , or (iii) - ‘our ow n view ’ - a new species, T. rh od esien se.
It is interesting that, even at the outset, there should have b een som e doubt in the minds o f the describers
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5J. R. BAKER
o f T. rh o d e s ie n s e about its validity as a separate spe
cies. T h ese doubts have survived until the present day, with scientific opinion swaying first one way and then another.
HOW MANY SPECIES OF 'POLYMORPHIC' TRYPANOSOMES IN AFRICA ?
There w ere two early schools o f thought. O ne, which might be called the ‘British’ school, concluded that T.
b r u c e i and T. r h o d e s ie n s e w ere ind istin gu ishable, though they recognized that not all ‘strains’ o f T. b ru
c e i in fecte d h um ans (K in g h o rn and Y o rk e , 1912;
Bruce et al., 1913). The opposite view, that the two w ere distinct species, was held by a group o f German w o rk e rs (T a u te , 1 9 1 3 ; K lein e, 1914, 1923; B e c k , 1914), w hose view was supported by the indisputable fact that six isolates o f T. b r u c e i w ere later show n by direct experim ental inoculation not to be infective to hum ans (T a u te and H uber, 1919 , w ho inclu d ed them selves am ong the 131 recipients o f trypanosome- infected blood).
Duke (1921) adhered to the ‘British’ view, suggesting that T. ‘r h o d e s ie n s e ’ and T. b r u c e i’ w ere merely ‘bio
logical races’ o f T. bru cei, and W enyon (1926), in his great m onograph, discussed the d ifferent opinions and concluded that, while T. g a m b ie n s e was a distinct species, ‘ T. r h o d esien se is merely a strain o f T. b ru c ei in m an.’
G radually, ho w ever, the ‘sep aratist’ G erm an view becam e generally accepted. The question seem ed to be settled by the historically fam ous ‘Tinde exp eri
m ent’. This experim ent, w hich was conducted by a series o f w orkers at the small experim ental station at T in d e in n o rth e rn T a n z a n ia ( th e n T a n g a n y ik a ), consisted o f the serial passage o f a strain o f trypano- som es, isolated in 1936 from a human patient, alter
nately through G lo ssin a m o rsita n s and sheep, with periodical testing o f its infectivity to humans either by blood inoculation or tsetse fly bite. The experim ent continu ed for 23 years, and the strain, w hich was considered to be T. rh o d esien se, never lost its ability to infect humans (apart from a few sporadic failures, a ttr ib u te d to fa u lts in t e c h n iq u e ) (W ille tt an d Fairbairn, 1955; Ashcroft, 1959).
With hindsight, it is easy to regret that the converse experim ent - serially passaging a strain o f T. b ru c ei k now n not to be infective to hum ans, through G.
m o rsita n s and sheep - was not conducted simulta
neously. As pointed out by Rickman (1977), it would have b een interesting to see w hether such a strain w ould have sp o n tan eo u sly acq u ired the ability to infect humans.
For many years epidem iological evidence had been building up to suggest that hum an-infective strains o f T. b r u c e i sen su la to existed in wild 'gam e' mammals, and this was conclusively demonstrated w hen Heisch et al. (1958) isolated such a strain from T rag elap h u s scrip tu s in Kenya. Eight years later, O nyango et al.
(1966) isolated a similar strain from a dom estic cow in the same country. However, this was not perceived as undermining the current concep t o f the distinction o f the two ‘sp ecies’ - T. b r u c e i and T. rh o d esien se. In the book T h e African trypanosom iases’ (Mulligan &
Potts, 1970) - in many w ays a sum m ing-up o f the British con tribu tion to trypanosom iasis research in Africa during the then rapidly ending ‘colonial’ era - Hoare (1970a) referred to the three species T. bru cei, T. rh o d esien se and T. g a m b ie n s e , although he quali
fied this by adding that, ‘from the zoological point o f view ’, they could b e classified as su bsp ecies o f T.
b r u c e i and in the next chapter o f the sam e b o o k he wrote that T. rh o d e s ie n s e represented ‘merely a viru
lent race o f T. g a m b ie n s é (see Hoare, 1970b). In 1925 (quoted by Hoare, 1972) and in 1943 Hoare had sug
gested that T. r h o d e s ie n s e ’ and T. ‘g a m b i e n s e ’ w ere merely races o f T. bru cei. He later (Hoare, 1966) ele
vated them to subspecies. In 1970 (Hoare, 1970a, b) his ideas seem have to b e e n in a fluid state, but finally, in his ‘magnum opus’, Hoare (1972) no longer treated T. r h o d e s ie n s e as a distinct su bsp ecies, but merely as one o f two ’nosodem es' o f T. b r u c e i g a m bien se.
In 1967, hum an African trypanosom iasis w as newly reported in south-western Ethiopia and, as a result o f investigations there, my colleagu es and I (B ak er et al., 1970; McConnell et al., 1970) concluded that the disease was, indeed, a new occurrence in that area.
In discussing its origin, w e considered three possibili
ties: that it had always b een covertly present; that it had b een new ly introduced from endem ic areas to the south; or that it had arisen from enzootic T. b r u c e i which had ‘recently acquired the ability to infect m an ’. Although w e regarded the last possibility as
‘in te re stin g ’, w e w ere rath er d ism issiv e o f it and concluded that introduction from the south was the m ost likely ex p la n a tio n (th e q u o ta tio n s are from Baker et al., 1970).
However, the idea that these trypanosom es need not necessarily be categorized into rigid divisions began to take root. Hoare (1967) suggested that the T. b r u c e i group, w hich he had redefined (Hoare, 1964) as fo rm in g th e su b g e n u s T r y p a n o z o o n Lü h e, 1906, should be regarded as ‘com plexes o f morphologically id e n tica l p o p u la tio n s ’ o r, in th e te rm in o lo g y o f Huxley (1963), ‘polytypic sp ecies’. This concep t was developed, independently, by two Canadian workers
6
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Parasite, 1995, 2, 3-12(Bardsley and Harmsen, 1973) w ho w ere studying the tryp anosom es o f anuran am phibia. T h ese w orkers conclu ded that the trypanosom es o f Anura form ‘a sp ecies co m p lex co m p osed o f a num ber o f clonal aggregates o f varying degrees o f separateness [one or tw o o f which] can be recognized as separate sp e
c i e s ...’; th ey ad d ed th e cau tio n ary n o te that ‘W e must... be prepared for the possibility o f finding one or m ore central cores o f highly variable, polymorphic species which may have to remain as unresolved spe
cies co m p lex es in the eyes o f all ex cep t the m ost ardent “splitters’”. This is a co n cep t w hich I think should be borne in mind w hen considering the sub
genus T ry p a n o z o o n today, uncom fortable though it may be for ardent ‘splitters’.
Experimental investigation o f the interrelationships o f the T. b ru c e i com plex was bedevilled for many years by the lack o f any means o f determining the ability, or inability, o f a particular strain to infect hum ans other than its experim ental inoculation into a human volunteer. The situation changed w hen Rickman and Robson (1970) described the ‘blood incubation infec- tivity test' (B IIT). If trypanosom es o f T. b n ic e i sen su la to w ere incubated under controlled conditions with human blood (or serum), som e strains lost their infec- tivity for rats while others retained it; and the pheno
m enon appeared to correlate well with their inability or ability to infect human beings.
It was show n by Rifkin (1978) that the trypanocidal factor w as associated with the serum high density lipoproteins (HDL), and later work has show n it to be located in several different HDL populations (Lorenz et al., 1994). Studies by Hajduk and his collaborators (H ag er et a l., 1994; Hajduk et a l., 1994; see also Rowe, 1994) have show n that the lytic activity resides in a m inor subspecies o f HDL. This com ponent may bind to a surface receptor in the flagellar pocket o f T.
b n i c e i and su bsequ en tly en ter the p arasite’s lyso- somes. The lysosomal m em branes are then disrupted, resulting in autodigestion o f the parasite cell. Why this does not occur in the hum an-infective trypano
som es is currently being investigated.
At first, the B IIT seem ed to support the distinction b e tw e e n T. b r u c e i and T. r h o d e s ie n s e . H ow ever, more detailed studies by Rickman (1977) show ed that different antigenic variants o f a strain initiated by ino
culating a single trypanosom e into a rat (a cloned strain) could show different BUT responses, sugges
ting that the property o f infectivity or noninfectivity to humans was not necessarily fixed. This led Rickman (1977) to suggest that hum an-infective trypanosomes could ‘em erge’ am ong a population o f hitherto nonin- fective (serum sensitive) individuals.
Loss o f human infectivity by strains o f T. rh o d esien se
during serial non-cyclical passage (i.e., passage by syringe inoculation) through laboratory rodents had long b e e n k now n to o ccu r. W hat w as surprising, how ever, was that Rickman’s work (1977) seem ed to show that the change could occu r in the opposite direction - from not infective to infective; in other w ords, that T. b r u c e i cou ld spontaneously change into T. rhod esien se.
This heretical idea w as given som e support by the fact that, in 1975, a m edical student w orking in a laboratory in Edinburgh, Scotland, becam e acciden
tally infected with a strain o f T. b r u c e i sen su la to w hich had been isolated from a G lossin a p a llid ip e s in south-east Uganda 15 years earlier (Robertson et al., 1980). This strain had b ee n frequently handled in laboratories without special precautions, and no acci
dental infection had occurred; it was therefore assu
m ed to b e n o n -in fe c tiv e to h u m ans - alth o u g h , unfortunately, this had never been deliberately tested.
A cloned strain had b een prepared from a single try
panosom e o f the original isolate, and from this 12 dif
ferent variable antigenic types had b een selected and cryopreserved. A serological study o f the infected stu
dent show ed that he had b ee n infected with type ETat 10 (ETat = Edinburgh trypanosom e antigenic type) (Herbert et al., 1980), which was found to be the only one o f this series o f 12 which was resistant to human serum in vitro (Van M eirvenne et al., 1976), and therefore, presumably, the only one to be infec
tive to humans. Happily, the unfortunate student fully reco v ered after treatm ent w ith suram in. T h e m ost likely e x p la n a tio n o f this ev en t seem s to b e that human infectivity had b een spontaneously developed by ETat 10, possibly by mutation, since the other ele
ven types (all o f which had originated from the same trypanosom e) w ere rendered non-infective by human serum.
EVIDENCE FROM RECENT MOLECULAR AND GENETIC STUDIES
The modern, m olecular approach to a study o f rela
tionships within the subgenus Try p a n o z o o n ( T b n i c e i sen su la to) originated with the painstaking studies by Godfrey and his co-w orkers o f the isoenzyme pro
files o f about o ne thou sand trypanosom e p opu la
tions, as revealed by starch gel and cellulose acetate electrophoresis. The enorm ous amount o f data thus produced was then analysed with the aid o f a com puter to determ ine the degree o f relatedness o f the v ario u s iso e n z y m ic g ro u p s (te rm e d zy m o d em es;
W H O , 1 9 7 8 ). T his w ork, w h en first rev iew ed by Gibson et al. (1980), indicated that there could be ‘no d oubt... that T r y p a n o z o o n stocks form a h om oge-
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7J. R. BAKER
neous group [and] that T. b ru cei, T. r h o d esien se and T. g a m b ie n s e are too closely related to deserve sepa
rate species status...’ (G ibson et al., 1980, p. 199). The study also strongly suggested that T. ev a n s i - a ‘spe
cies’ not transmitted by G lossin a and chiefly associa
te d w ith c a m e ls - s h o u ld b e in c lu d e d in th is
‘hom ogeneous group’. G ibson et al. (1980) proposed d istin guishing six su bgro u p s, to w hich they gave names without taxonom ic significance. O ne o f these groups ( ‘gam biense’) correlated fairly well with the
‘c la s s ic a l’ T. g a m b i e n s e , but no o n e o f th e o th er groups could be related clearly with the classical ‘ T.
r h o d esien sé or ‘ T. b r u c e i.
A later, shorter review by G ibson (1986) o f this and other work concluded that there did exist a group o f stocks which corresponded more or less to the classi
cal concept o f ‘T. g a m b ien se' as infective to humans, o f low virulence to experim ental animal hosts, and generally confined to Central and W est Africa. This group G ibso n (1 9 8 6 ) referred to as ‘group 1 T. b.
g a m b ie n s é ; she also recognized a ‘second, less easily defined group w ith greater... h etero g en eity ’ w hich she provisionally referred to as ‘group 2 T. b. g a m b ie n s é . Group 2 had b een isolated only from Burkina Faso and Côte d’Ivoire.
Summarizing and extending this work, Godfrey et al.
(1 9 9 0 ) and Stevens and G odfrey (1 9 9 2 ) concluded that most or all o f the zymodemes o f T ry p an ozoon , w h en num erically analysed by the construction o f both a dendrogram and a cladogram, fell into three m ajor divisions or pathways, w hich corresponded ,
‘albeit im perfectly’. These divisions did not, however, correspond with the three classical species or subspe
cies, but w ere separated more on geographical crite
ria. O ne division was predominantly East African, one was mainly W est African, and the third formed a ‘resi
dual g ro u p ’ lin ked w ith the Lake V ictoria region.
Using the a cc e p te d n o m en clatu re , G od frey e t a l.
(1990) concluded that ‘ T. e v a n si and T. b. g a m b ie n s e appeared to be distinct entities.... The status o f T. b.
rh o d e s ie n s e and T. b . b r u c e i was particularly uncer
tain...’. These authors finally concluded that it may be
‘reasonable’ to retain the name T. b. r h o d esien se for their W est African strain groups, w hether or not they are infective to humans, and the name T. b. b r u c e i for an essentially W est African strain group, pred om i
nantly but not exclusively non-infective to humans.
These conclusions have b een broadly supported by the work o f other authors. Tait et a l. (1984, 1985), also using isoen zym e electro p h o resis as a to o l to investigate the com plex relationships within the sub
genus T ry p an oz oo n , concluded that T. b. g a m b ie n s e was a valid subspecies w hich could be distinguished from other stocks o f the subgenus which they refer
red to as ‘ T. b r u c e i (n o n -g am b ien se )’ (in the 1984 paper). They concluded that the so-called T. b. r h o d e s ie n se w as ‘a set o f variants’ o f T. b. b r u c e i rather than a true subspecies (Tait et al., 1985).
Paindavoine et al. (1986, 1989) exam ined the deoxyri
bonucleic acid (DNA) o f som e 70-80 populations o f Try p a n o z o o n by means o f digestion with restriction en d o n u cle ases, h ybrid ization and electro p h o re sis.
These authors concluded that all the T. b. g a m b ie n s e stocks w hich they exam ined had a conserved, speci
fic DNA band pattern and could be ‘non-am biguously identified’, form ing a single h o m ogen eou s p opu la
tion. T he other stocks they exam ined, in contrast, yielded highly variable DNA patterns and w ere distri
buted am ongst several h eterogeneou s groups. T. b.
g a m b ien se , they concluded, was therefore a ‘real sub
species’ w hereas ‘ T. b. rhodesien se' was not, and they suggested that the h eterogeneity indicated that the
‘n on-gam biense’ trypanosom es w ere evolving m ore rapidly than T. b. g a m b ie n s e itself (Paindavoine et a l ., 1986). In their later paper (Paindavoine et al., 1989), these authors reported that studies using four DNA probes had show n that all 49 T. b. g a m b ie n s e stocks exam ined possessed the same com bination o f alleles and w ere hom ozygous for the four m arkers used.
This study supported the view that the ‘n on-gam b ien se’ stocks w ere diploid and could undergo g ene
tic exchange (see below ). T. b. g a m b ie n s e , however, they thought w as g en etically isolated and did not exch ange g enetic m aterial w ith other stocks o f the subgenus T ry p a n o z o o n . This study also supported the claim by Dero et al. (1987) that the genom e o f T.
b. g a m b i e n s e c o n ta in e d o n ly a b o u t 70% o f th e amount o f DNA in the genom e o f the non-g a m b ie n s e stocks, and only about 50% o f the latter’s variable antigen gene repertoire.
CLONAL OR SEXUAL POPULATION STRUC
TURES ?
I
n spite o f sporadic publications claiming to have d etected sex u al rep ro d u ctio n am ong tryp an o som es (review ed by Hoare, 1972, pp. 48-51), it was generally accepted until about 15 years ago that genetic exchange did not occur am ong trypanosom es o f the subgenus T ry p a n o z o o n (nor, indeed, am ong any other groups). However, in 1980 Tait again raised the possibility o f genetic exchange occurring, supporting his claim with evidence o f apparent hybrid for
mation detected by isoenzym e electrophoresis.
T h is w o rk h as b e e n c o n s id e ra b ly e x te n d e d and confirm ed by several w orkers (for exam ple, Jen n i et a l., 1986; Schw eitzer et a l., 1988; Sternberg e t a l.,
Mise au point
Parasite, 1995, 2, 3-12 81988, 1989; Gibson, 1989; Paindavoine et a l., 1989;
P earson & Jen n i, 1989). It now seem s indisputable that som e form o f genetic exchange can occur during that part o f the life cycle which takes place within G lo s sin a , at som e stage b efo re the p rod u ction o f m etacyclic trypom astigotes. T he details o f the pro
cess, however, are unclear, and - perhaps suprisingly - no confirmatory cytological evidence o f the process has yet b een obtained.
It is also clear that this genetic exch ange is not an obligatory process and, although it may b e frequent b etw een certain stocks (Stern berg et a l., 1989), its occurrence is by no m eans universal. It has b een sug
gested by Pearson & Jen n i (1989) that som e system o f mating types may be involved, as is well know n to be the case with P a ra m ec iu m . The DNA o f the kine- to p la st a p p e a rs n o t to tak e part in this p ro c e ss;
Sternberg et al. (1988) show ed that inheritance o f the kinetoplast was uniparental.
T h e re a lso se e m s little d o u b t that try p a n o so m es ( T ry p a n o z o o n ) are diploid throughout most o f their life cycle. This was first suggested by Tait (1980), and confirm ed by G ibson et al. (1985), Paindavoine et al.
(1989) and Pearson & Jen n i (1989).
How ever, Cibulskis (1988), w hile agreeing that the
‘range o f genotypes in T. b ru c ei is most readily explai
ned by genetic exchange’, rightly pointed out that the frequency o f its occurrence was (and is) not known; it
‘may not occur sufficiently frequently, or in such a w ay as to b re a k up a s s o c ia tio n s b e tw e e n l o c i’.
Cibulskis (1988) concluded that ‘it is not clear that sex exerts any control over the relative proportions o f genotypes at individual lo c i.’ This con clu sion is in accord with the views o f Tibayrenc and his co-w or- kers (Tibayrenc et al., 1990, 1991; Truc & Tibayrenc, 1993; Mathieu-Daude & Tibayrenc, 1994) who, from a study o f population genetics, concluded that trypano
som es (and many other parasitic protists) have an essentially clonal population structure, rather than one which is based on sexual reproduction; their analysis has shown that segregation and recombination, which are necessary con sequ ences o f sexual reproduction, are rare or absent in natural populations o f the orga
nisms concerned. This does not exclude the possibi
lity that sex u al p ro c e sse s m ay o ccu r, bu t m erely indicates that they may occur infrequently and are not generally o f genetic significance in these populations (T ib a y re n c e t a l ., 1 9 9 0 , 1 9 9 1 ; M ath ieu -D au d e &
Tibayrenc, 1994; Mathieu-Daude et al., 1994).
A consequence o f this view is that the clones, many o f which appear to be stable over space and time, are the basic taxonom ic unit rather than the conventional Linnean species and subspecies. The three ‘subspe
cies’ o f T. b r u c e i w ere regarded by Tibayrenc et al.
(1990) merely as ‘pathotypes’, the species being com p o sed o f ‘num erous clo n es, som e o f w h ich have beco m e specialized to hum an ho sts’. The T. g a m - b ie n s e group 1 o f G ib so n (1 9 8 6 ) is a ‘g en etically h o m o gen eo u s... su ccessful, ubiquitous hum an-host c lo n e ’ (T ib a y re n c e t a l ., 1 9 9 0 ). In a la ter p ap e r (M athieu-Daude and Tibayrenc, 1994) this group o f w orkers used the ‘d em e’ terminology introduced into the trypanosom iasis literature by Hoare (1955) and referred to the so-called T. b. b r u c e i and T. b. rh o d e- s i e n s e as n o s o d e m e s ’ rath er th an actu al g e n e tic clades; T. b. g a m b ie n s e group 1 (Gibson, 1986) was regarded as a ‘group’ o f clones [not, now, a single clone] consisting o f m ost o f the stocks o f hum an- infective trypanosom es from Central and West Africa.
In another paper, Mathieu-Daude et al. (1994) sho
w ed that a group o f 12 stocks o f human-infective try
panosom es from Central and W est Africa w ere closely related by the use o f a specific kinetoplast deoxyribo
nucleic acid probe, and they identified this group as G ibson’s (1986) group 1 T. b. g a m b ien se.
Truc and Tibayrenc (1993) also equated one group of zymodemes, among the 23 w hich they identified in 55 stocks o f T. b r u c e i s e n s u la t o isolated in W est A frica, w ith th e ‘c la s s ic a l’ T. b. g a m b i e n s e . T h e various zymodemes, they suggested, w ere equivalent to ‘natural c lo n e s (o r a fam ily o f c lo se ly related clones), stable in space and tim e.’
CONCLUSIONS
T
he inescapable conclusion from the foregoing brief review seems to me to be the fact that it is futile to attempt to maintain the distinction betw een the erstwhile subspecies (or species) T. b.b ru c ei and T. b. rhodesien se. These organisms must be regarded as a collection o f populations (whether clo- nally or sexually derived), all fairly closely related - Mathieu-Daude & Tibayrenc (1994) pointed out that the total genetic variability detected in a study o f 18 isoenzyme loci amongst 78 stocks o f T. b ru c ei sen su lato was less than that seen in the ‘single’ species T.
c r u z i - some o f w hich had developed the ability to resist destruction by HDL in human blood; this distinc
tion, though often durable for quite long periods, can
not be thought o f as immutable. I am not in favour of the proposal by Godfrey et al. (1990) to redefine T. b.
b ru c ei and T. b. rh o d esien se in terms other than their infectivity (o r non-infectivity) to human hosts. This course o f action would, I think, be confusing since the two subspecific names have for so long been used to denote precisely that characteristic.
The situation is different with T. b. g a m b ien se . All the recent w ork discussed above has show n that there
Parasite, 1995, 2, 3-12
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9J . R. B A K E R
e x ists a c o re o f ‘ty p ic a l’ h u m a n -in fe ctiv e T ry p a - n o z o o n stocks producing chronic human African try
panosom iasis, based on (but not restricted to) West Africa, w h ich can fairly certainly b e distinguished from other stocks o f the T. b r u c e i com plex. The argu
ment has thus, in a sense, swung around a full circle : T. rh o d e s ie n s e is not a valid species, or even subspe
cies, not b ecau se it is indistinguishable from T. b.
g a m bien se, as the early British workers thought, but b e c a u s e it is m erely a g ro u p o f zy m o d em es (o r clon es) o f T. b. bru cei.
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