Genome size variation and evolution in North American cyprinid fishes

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Genome size variation and evolution in North American

cyprinid fishes

Jr Gold, Cj Ragland, Lj Schliesing

To cite this version:

Original

article

Genome size

variation and evolution

in North American

_cyprinid

fishes

JR

Gold

CJ

_Ragland,

_{LJ Schliesing}

Department

of

Wildlife and Fisheries _Sciences, Texas A & M

_University,

College

Station, TX _77843, USA

(Received

4

_January

1989;

accepted

11 October

₁₉₈₉₎

Summary - Genome sizes

_(nuclear

DNA

_contents)

were documented

spectrophotomet-rically for 29 _species of North American _cyprinid fishes. The data were then

_merged

with _{comparable genome} size data

_{(published previously)}

from an additional 20 North American _{cyprinid species.} The distributions of DNA values within _populations of the 49

cyprinid species were _essentially continuous and normal. The _proportion of DNA which

apparently is free to vary quantitatively within _{cyprinid populations appears} to be

be-tween 4 and 5 % of the genome. The distribution of DNA values among cyprinid species

was more-or-less _continuous, with considerable _{overlap among species} with intermediate DNA values.

_Analysis

_{of the average genome} size difference

_(distance)

between individuals drawn from successive levels of _evolutionary

_divergence

indicated that:

_(i)

the _majority of

genome size divergence in North American _{c prinids} has occurred above the level of indi-viduals within _populations of _species, and

(ii)

the

_degree

_{of genome size}

_divergence

in the

extremely speciose cyprinid genus Notropis is greater than that between _species in _other, less _{speciose cyprinid genera.} The _{hypothesis that genome size}

_{change might}

be

concen-trated in _{speciation episodes} was tested

_{by comparing}

the means and variances of

_genome

size difference

_(distance)

between _species in the _cyprinid genus Notropis

(a

species-rich

phylad)

and the centrarchid

_(sunfish)

genus Lepomis

(a

species-poor

phylad).

The ratios

of mean distances and variances in the Notropis versus _{Lepomis comparisons} were _greater than _{unity, suggesting} that _changes in _{genome size in cyprinids may} be correlated with

speciation episodes. Whether or not _{genome size change} in _cyprinids occurs at _speciation sensu strictu is _problematic. The data _suggest that separate facets or levels of the cyprinid

genome may follow independent

evolutionary

paths.

genome size (DNA

content)

_{/ cyprinid fish / natural selection / speciation}

Résumé - Variation et évolution de la taille du _génome chez les _{cyprinidés d’Amérique}

du Nord - La taille du

_génome

_(estimée

_par la _quantité d’ADN

_nucléaire)

de 29

_espèces

Nord-Américaines de _cyprinidés a été mesurée _{par spectrophométrie;} les résultats ont

ensuite été _jumelés à des données _{comparables publiées antérieurement,} obtenues sur 20

autres _espèces de _cyprinidés de la même aire _{géographique,} et les _analyses ont été conduites sur l’ensemble de ces données. Au sein des populations, la _quantité d’ADN nucléaire suit

une distribution continue et _normale, et varie dans une _{proportion qui représente 4} à 5% du génome. Etudiée sur l’ensemble des _espèces, la _quantité d’ADN nucléaire _présente une distribution _{quasiment continue,} avec des chevauchements considérables entre _espèces.

*

L’analyse de la variation observée _parmi des individus tirés dans des niveaux taxonomiques

variés _indique que:

- la variation est essentiellement due aux variations entre

espèces et non aux variations

entre individus d’une même _espèces, et que,

- la variation entre

espèces est _plus étendue dans _{le genre Notropis que} dans d’autre

genres moins

diversifiés.

L’hypothèse selon laquelle les modifications de taille génomique

seraient concentrées à l’occasion _{d’épisodes} de _{spécification} a été testée en, comparant, dans _groupes

_différant

par leur degré de

_{différentiation,}

les _{moyennes et} les variances des écarts constatés entre les _{différentes espèces} au sein de chaque groupe: le genre Notropis

(cyprinidés),

phylum

riche en _espèces, et le genre Lepomis

(centrarchidés, poissons-lunes),

phylum

pauvre en _espèces. Les _rapports des _{moyennes et} des variances de _Notropis

comparées à _Lepomis sont tous _{2 supérieurs} à _l’unité, ce _{qui suggère} une corrélation

entre les variations de taille _génomique et les _épisodes de spéciation chez les _cyprinidés; la relation exacte entre de tels _changements et la _spéciation sensu stricto demande _cependant à être _précisée. Les résultats _{suggèrent enfin} que les processus évolutifs sont _susceptibles

de

_différer

en _fonction des

_{facettes envisagées}

du _génome des

_cyprinidés.

taille du génome (quantité d’ADN) / cyprinidés / sélection naturelle _/ spéciation

INTRODUCTION

It has been known for several years that sizeable differences in genome size or DNA

content often occur, even between

_closely

related

_species

(Mirsky

and

_{Ris, 1951;}

Bachmann et

_al,

_1972;

_Sparrow

et

_al,

1972. Kauffman

₍₁₉₇₁₎

_{initially hypothesized}

that the _{extensive genome size variation} was related

_directly

to

_organismal

_and/or

genetic complexity.

It is now

_{clear, however,}

that no

_significant

correlations exist between _{genome size and}

_organismal

_{(or genetic)}

_complexity

or

_phylogenetic

advancement

_{(Cavalier-Smith,}

_{1985a; Price,}

_1988a).

This has been termed the C-value

_paradox

and represents a

_{general biological problem}

_among

_eukaryotes

which to date remains unresolved

_{(Price, 1988a,b,c).}

Efforts towards

_explaining

or

understanding

the C-value

_paradox

have been

focused

_primarily

on the search for

_significant

correlations between _{genome size}

and a

_variety

of

biological,

biophysical

or

_genetic

_parameters. What has

_emerged

from these studies are several

_hypotheses

which relate genome size in an inverse

way to rates of

_{organismal growth,}

metabolism or

differentiation,

and which invoke

selection as the

_primary

force

_responsible

for the observed variation in genome size

(Bennett,

1971, 1972;

Cavalier-Smith, 1978, 1980, 1985a, b; Szarski,

1983;

Sessions

and

_{Larson, 1987;}

Price

_1988a).

These

_hypotheses

are confounded for several reasons.

_First,

much of the data

which document

_{relationships}

between _{genome size} and cell

_cycle

_patterns or certain

life

_history

_parameters are from unicellular

_eukaryotes

_(eg,

_{Cavalier-Smith, 1980;}

Shuter et

_al,

_1983).

The

_problem

lies in the

_{extrapolation}

to multicellular

_eukaryotes

where it is often dif6cult to obtain

_direct,

unbiased or standardized estimates of

organismal growth

and/or

developmental

rates. A second reason is that _most, if not

_all,

of the evidence is correlative and does not

_necessarily

demonstrate

cause and effect. A third reason is that

_nearly

all of _{the genome size data} are

from distinct

_species

or

_higher

level taxa. Studies of _{genome size variation} at lower hierarchical levels are

_few,

and differences _{in genome size} within

_species

generally

have been

_regarded

as

_{insignificant}

or

_unimportant

_(Bennett

and

_Smith,

genome size may be

substantial,

and in some cases

_approximate

the _{average genome}

size differences observed between

species

(Price

et

_{al, 1981, 1986;}

Sherwood and

Patton,

1982;

Gold and

_Price,

_1985;

Gold and

_Amemiya,

_1987;

Johnson et

_al,

1987;

Ragland

and

_Gold,

_1989).

A final reason is that little attention has been

paid

to the mechanisms

_by

which DNA

_might

be

_gained

or lost from a _genome.

The observations that

species

within cohesive

_groupings

_{(eg, genera)}

often differ

substantially

in genome size and that

_interspecies

_{genome sizes} are

_frequently

discontinuously

distributed have led to the

_suggestion

that _{genome size evolution} may occur in a

&dquo;quantized&dquo;

_{fashion; ie,}

_by

a succession of

_large-scale

_changes

(Narayan,

1982;

Cavalier-Smith,

1985b).

Subsumed within this

_problem

is the

question

of whether _genome size

_changes

_might

be

_{occurring disproportionally}

during

speciation episodes.

Several authors

_{(Hinegardner,}

1976; Morescalchi, 1977;

Cavalier-Smith,

1978)

have

_suggested

that genome size

change might

be associated with

_speciation,

_although

a direct correlation between genome size

change

and

speciation

has not been tested

_critically.

In the

_following,

data on intra- and

_{interspecific}

_{genome size variation among} 49

_species

of North American

_cyprinid

fishes are

_presented.

The genome size data from 29 of the

species

are

_given

for the first time. The

_subjects

of

_primary

interest

in the _paper are:

(i)

the _pattern and

_magnitude

of _genome size variation within

populations

and among

species,

and

_(ii)

the

_question

of whether _{genome size}

changes

are concentrated in

_speciation

_episodes.

MATERIAL AND METHODS

The collection localities of

_{samples representing}

the 29 North American

_cyprinid

species,

whose _{genome sizes} are

reported here,

are

_given

in the

Appendix,

Table A1.

All fish were collected

_by

seine from natural

populations.

Fish

_sampled

from Texas

(TX)

and Louisiana

_(LA)

were returned live to our

laboratory

in

College

Station for

processing;

fish

_sampled

from Oklahoma

_(OK)

and Alabama

_(AL)

were

processed

in facilities at the Oklahoma

_University

_Biological

Station on Lake Texoma and at Samford

_University

in

_{Birmingham, AL, respectively. Except}

for

_{Notropis lepidus,}

the

_samples

of each

_{species comprised}

5 individuals taken from the same

locality.

The N.

_{lepidus sample comprised}

10 individuals from the same

_locality.

Collection

localities for the 20 other North American

_cyprinid

species

included in the data

analyses

in this _{paper, may} be found in Gold and

_Amemiya

_(1987).

In that

_study,

the

_samples

of each

_{species comprised}

10 individuals taken from the same

_locality.

Genome sizes were measured via

_scanning

_{microdensitometry}

of

Feulgen-stained

erythrocyte

nuclei

_using

chicken blood as an internal control. The latter was

obtained from a

_highly

_{inbred, pathogen-free}

strain available from the Texas A & M

_College

of

_Veterinary

Medicine. Full details of slide

_preparation,

_staining

and

microdensitometry

may be found in Gold and Price

(1985)

and Gold and

Amemiya

(1987).

Fifteen

_erythrocyte

nuclei were measured from each of 2 slides _per fish

(=30 nuclei/individual)

and standardized as a _percent of the mean

absorbancy

of

10

chicken

erythrocyte

nuclei on the same slide. Standardized

absorbancy

values of

fish nuclei were coded

_(for

_convenience)

_by

_multiplying

the _percent chicken standard

Means,

standard errors and ranges for the 29

species

were taken from the

dis-tribution of DNA values of individuals within each

_species.

Distribution

_normality

indices

_(g

_l

and

_g

₂

₎

were taken from the distribution of measurements

_(nuclei)

within each

_species.

_Descriptive

statistics of genome size variation within and among the 20

_{cyprinid species}

not

_reported

here _may be found in Gold and

_Amemiya

_(1987).

The

_{methodologies}

used to determine _genome sizes of individuals in all 49

_species

were identical. The current classification of the 49

_species

is shown in the

_Appendix,

Table All. Note that 31 of the 49

_species

are from the

_{extremely speciose}

_genus

Notropis

which includes over 125

_species

(Lee

et

_al,

_1980).

RESULTS

Descriptive

statistics

_{(means +}

standard errors, ranges and the gi and g2 indices of distribution

_normality)

for the 29

_species

are

_given

in Table I. Genome sizes

ranged

from 2.06 pg of DNA in

_Notropis

callistius to 3.26 pg of DNA in Phenacobius

catostomus, a difference of

_{approximately}

58%. The ranges of _genome sizes within

each of the 29

_species

varied in _percent from 1.15 in

Notropis

beldus to 8.74 in Dionda

_episcopa,

and

_averaged

4.11. Five of the 29

_sampling

distributions of measurements

_(nuclei)

within each

_species

were

_{significantly}

non-normal. Of the

5,

3 were

_{significantly platykurtic}

or

_flat,

and 2 were

_{significantly}

skewed towards

higher

DNA values.

Patterns and

_magnitude

_{of genome size} variation within

_populations

of

_species

The coded

_absorbancy

data from the 49

_{cyprinid species}

examined to date were

organized

into a number of different

sampling

distributions and each was tested

for distribution

_{normality using}

the _gl and ₉₂ indices. The distributions tested included:

_(i)

all measurements

_(nuclei)

within each

_population

_(species)

or

_sample

(49

sampling distributions;

N = _{300 for}

_populations

where 10 individuals _were

examined and N = 150 for

populations

where 5 individuals were

_sampled);

and

(ii)

a rankit distribution

_(Sokal

and

_Rohlf,

₁₉₆₉₎

_reflecting

the distribution of DNA values of individuals within

_populations

summed over all 49

_populations.

The latter was

_generated

_following

_eqn[l]

in Gold and

_Amemiya

₍₁₉₈₇₎

in order to remove

scaling

effects due to individuals

_being

drawn from different

_species.

The results of the distribution

_normality

tests are summarized in Table II. The

_majority

of the

distributions of measurements

_(nuclei)

within

_populations

were

_normal,

_although

the incidence of non-normal distributions was

_higher

than

_{expected by}

chance at

a = 0.05. The rankit distribution

reflecting

the distribution of DNA values of

individuals within

_populations

was

_{significantly}

_{platykurtic, although}

the deviation

appears

slight

(Fig 1).

Separate single

classification

_analyses

of variance

_(ANOVA)

were used to test for

_{significant heterogeneity}

of DNA values of individuals within each of the 49

_populations

_(species)

_using

the distribution of measurements

_(nuclei)

of that

species.

All F-tests were

_significant

at a = 0.05. A

_synopsis

of the results of

Duncan’s

_multiple

range test on each

population

is shown in Table III. The results

within

_{cyprinid populations}

and

_that,

on _average,

approximately

half of the

The

_magnitude

of genome size variation within

cyprinid

populations

was

esti-mated as the average of the _percent maximum variation between individuals within

populations.

These values

_ranged

from 1.15% in

_Notropis

bellus

_(Table

_I)

to 13.49%

in

_{Notemigonus crysoleucus}

_(Table

3 in Gold and

_Amemiya,

_1987),

and

_averaged

4.86 f 0.31%

_{(Table II).}

Assuming

an _average North American

_cyprinid

genome size of 2.47 pg of

DNA,

this represents

approximately

0.12 pg or

about

1.1 x

loll

Patterns and

_magnitude

_{of genome} size variation among

species

A

_plot

of the distribution of DNA values of individuals examined from all 49

_species

is shown in

_Fig

2. With the

_exception

of the 2

species

of Phenacobius

_(cf

Table

_I),

the

interspecies

distribution of genome sizes appears continuous and

overlapping.

Single

classification ANOVA was used to test for

_significant

_{heterogeneity}

_{in genome} size

variation among

species using

this

_sampling

distribution.

_{Significant heterogeneity}

of mean DNA values at a = 0.05 _was found and the results of _a Duncan’s

multiple

range test are shown in Table III.

_Again,

with

exception

of the 2

_species

of

Phenacobius,

interspecies

genome sizes appear more-or-less

continuously

distributed

Two

_approaches

were used to examine the

_magnitude

of genome size variation among the 49

species.

The first was to _{carry out} a nested

_analysis

of variance

(Table IV)

which revealed

that,

although significant heterogeneity

in genome size existed at each

_experimental

level from between slides within individuals to _among

species,

the

_majority

_(>88%)

of the variation occurs _among

_species.

The second

approach

was to estimate the

_magnitude

of _{genome size} differences at

_ascending

taxonomic levels. This was

_accomplished

_using

eqns

[2]

and

[3]

of Gold and

Amemiya

(1987).

Briefly,

eqn[2]

generates a _{genome size} difference or distance

(GSD

m

i

n

)

value

between 2

_species

or taxa which represents the average of all

pairwise

differences in genome size between all individuals

sampled

from each taxon or

_species

(eg,

with

N = 10 individuals for each of 2

species,

there are 100

_possible

_{comparisons).}

The 48 x 49

GSD

m

in

distance matrix

_(which

includes 1176

GSD

m

i

n

values)

generated

from these calculations is not shown but may be

obtained,

from the first author.

Equation

[3]

generates a

GSD

m

i

n

value which _represents the average of all

possible

pairwise comparisons

between all individuals of _{any one}

_population

of a

species

(eg,

for N = ₁₀

_individuals,

there _are 45

possible

comparisons).

The

GSD

m

i

n

values

for all 49

_populations

_(species)

were then

averaged

to obtain an estimate of the average genome size difference or distance between individuals within

populations

of

_species.

It should be noted that both

GSD

min

values are minimum linear distance

metrics which underestimate the true distance if reversed or reticulated _patterns of

change

occur

(Sneath

and

Sokal,

1973).

The _{average genome size} difference

_(distance)

between individuals drawn from successive levels of

_evolutionary

_divergence

are shown in Table V. Estimates of

average genome size distances between

species

in

subgenera

of

Notropis

and between

species

in

_Notropis

_{and in other genera} were obtained from subsets of

GSD

m

i

n

values extracted from the 48 x 49

GSD

m

i

n

distance matrix. The average genome

computing

the _{average genome size} distance value for each

_subgenus

based on all

pairwise comparisons

between

species

in that

_subgenus,

and then

_averaging

these values over all

_subgenera.

The same method was used to estimate the average genome size distance between

species

in genera other than

Notropis.

The estimate for

_species

in

_Notropis

is

_simply

the average of all

pairwise

comparisons

among 29 of the 31 nominal

_{Notropis species}

examined. Both N atrocaudalis and N stramineus

were not included in the latter estimate since the

_phylogenetic

af6nities of these 2

species

may lie outside of

Notropis

(Mayden, 1989).

For similar reasons, N rubeldus and N

_baileyi

were not included in _{the genome size} distance estimate for the

Notropis

subgenus Hydrophlox

(Mayden

and

_Matson,

_1988).

The genus

Pimephades

was included in the genome size distance estimate for

species

within the genus

Notropis

since

_Pimephales

is now believed to be

_closely

related

_{phylogenetically}

to

certain

_lineages

within

_Notropis

_(Cavender

and

Coburn,

1986).

The estimate for

species

in the

family

is _{the average of all}

_{pairwise comparisons}

among all 49

species

examined.

As shown in Table

_V,

individuals drawn at random from a

_population

of

the same

_cyprinid

_species

will

differ,

on average,

by

0.388 genome size distance units

_{(approximately}

_{0.048 pg of}

_DNA);

whereas,

any 2 individuals drawn at random from 2 different North American

_cyprinid

_species

will

_differ,

on _average,

by

2.322 genome size distance units

(approximately

0.290 pg of

DNA).

This

represents a 6-fold difference and

strongly

_{suggests that} the

majority

of _{genome size}

divergence

in North American

_cyprinids

has occurred above the level of individuals within

_populations

of

_species.

_{Particularly noteworthy}

are the observations that

(i)

the

_degree

of genome size

divergence

between

species

in the genus

Notropis

is

_{approximately}

5 times that between

species

in other

_cyprinid

genera, and

(ii)

much of the

_divergence

in

_Notropis

has

_apparently

occurred at the

_subgeneric

rather than

_generic

level. The most

_{actively evolving}

Notropis

subgenera

in terms of _{genome size appears to} be

_Cyprinella

and

_Notropis,

where the _{average genome} size distance between

_species

was estimated as 2.152 and 2.340

units,

respectively.

Since these are the 2

largest

Notropis subgenera

in terms of number of

species,

and since

_Notropis

itself contains

_considerably

more

_species

than

Campostoma,

Nocomis or

_Phenacobius,

the tentative

implication

of these data is that there may be a

_{positive relationship}

between the number of

_species

within a _group or

_subgroup

Genome size

_change

and

_speciation

The

_findings

that the

_majority

_{of genome}

size variation in North American

_cyprinids

appears to occur at the

_species

level or

_above,

and that a

_relationship

may exist between the number of

_species

within

_cyprinid

_groups or

_subgroups

and

divergence

in genome

size,

suggest that genome size

changes

in

_cyprinids

may be concentrated

in

_{speciation episodes.}

Avise and

_Ayala

_(1975,

₁₉₇₆₎

and Avise

₍₁₉₇₈₎

_developed

models which contrast

_expected

means and variances of

genetic

differences or

distances _among extant members of

_rapidly

versus

slowly

speciating

lineages

or

phylads,

and which may be used to assess whether

_genetic

differentiation is

correlated with

_speciation.

_Briefly,

if

_genetic

differentiation is

_essentially

a function

of time

_(gradual

_evolution),

the ratio of mean

_genetic

distances between

species-rich

versus

_species-poor

phylads

should be

approximately

1,

and the ratio of variances

should be less than 1.

_{Alternatively,}

if

_genetic

_{differentiation}

is

_proportional

to the number of

_speciation

_episodes

_{(punctuated evolution),}

the ratio of distances should be _greater than

_1,

and the ratio of variances should be much _greater than 1. There are several

_assumptions

inherent in

_using

the

models,

the most

_important

of which is that the

_species-rich

and

_species-poor

_lineages

under

_comparison

be of

approximately equal

evolutionary

age

(Avise

and

Ayala,

1975;

Avise,

1978).

In Table

VI,

the mean

(d)

and variance

_(s

₂

₎

of average genome size differences

(distances)

among 32

Notropis

species

(including

the 3

_species

of

_Pimephales)

are

compared

with

_comparable

values from 8

_species

of the centrarchid

_(sunfish)

_genus

Lepomis.

The distance and variance values were

generated

as before

_(ie,

extracted from the 48 x 49

GSD

mi

l1

cyprinid

data

matrix,

and from a similar

_Lepomis

data

matrix described in

_Ragland

and

Gold,

1989).

For reasons noted

previously,

the

N atrocaudalis and N stramineus were not. For similar reasons

(GV

Lauder,

personal

communication),

Lepomis

gulosus

was not included in the calculations of

d

and s2

values for the _genus

_Lepomis.

As shown in Table

_VI,

the ratio of mean distances is

greater

than

_1,

and the ratio of variances is very much _greater than 1.

_According

to the

_models,

these results indicate that

_changes

_{in genome size in}

_cyprinids

are

correlated with

_speciation

_episodes.

In Table

_VII,

observed ratios of mean distances and variances for the

_comparison

_Notropis

versus

_Lepomis

and data from

_protein

electrophoresis

and

_{morphological}

measurements are

_compared

to those based on

genome size. Taken at face

_value,

the observed ratios

_suggest

that differentiation in structural _genes and

_morphology

occurs

primarily

as a function of

_elapsed

time.

DISCUSSION

The

_normality

_(or

near

normality)

of _{genome size} distributions within

populations

of

_{cyprinids strongly}

_suggests that DNA

_{quantity changes}

at this level are

small,

involve both

_gains

and losses of

_DNA,

and are cumulative and

independent

in

effect. This

_hypothesis

is based on the

assumption

that the variation follows the

identical _pattern of variation also occurs among

populations

of 9

species

of the North American centrarchid genus

Leporrcis

(Ragland

and

Gold,

1989).

Of

importance

is that no instance of a _quantum or

_{&dquo;quantized&dquo;}

_{(Cavalier-Smith, 1985b)}

difference

in _{genome size among} individuals has been found in the

_nearly

60

_populations

of

cyprinids

or centrarchids thus far studied.

Comparable

data from other

organisms

on _{genome size variation among} several individuals within

populations

are

_few,

and

are limited

primarily

to the extensive researches

_by

Price and

_colleagues

on the

plant

Microseris

_douglasii

_(Price

et

_al,

1981,

1986).

In M

_douglasii,

genome size variation is also continuous with no

_evident,

_large-scale

differences in genome size

occurring

among individuals within

populations.

There was an _apparent

tendency

towards

platykurtosis

in a few of the

cyprinid

populations

genome size

distributions,

including

the rankit

distribution,

which

re-flects the normalized variation of DNA values of individuals. Most of the deviations from

normality, however,

were

_slight

_and,

in the case of the rankit

values,

the

distri-bution

_only

became

_platykurtic

upon the addition of the 28

populations

(species)

reported

in this paper, where

sample

sizes were restricted to

_only

5 individuals per

population.

This suggests that the observed

_{platykurtosis}

may be a function of

non-random

_sampling

since

_typically

most individuals were collected in

only

1 or 2 seine-hauls and could _represent close relatives

_(eg,

_full-sibs)

rather than individuals drawn at random from

_population.

The

_proportion

of

_DNA,

which

_apparently

is free to _vary

_{quantitatively}

within

cyprinid populations,

appears to be between 4 and 5% of the _genome, as estimated

from the _{average maximum genome size variation among} all 49

_populations

sur-veyed.

This

_quantity

is

_{approximately}

the same as that

theoretically

needed for the

cyprinid

structural gene component if one assumes the latter contains 50 000

cod-ing

nuclear _{genes per} genome and there are 1500

coding

DNA base

pairs

per gene.

It seems

unlikely,

however,

that

coding

structural genes would be

regularly gained

or lost from a _genome without

_eventually

_resulting

in a

phenotypic

disturbance or

developmental irregularity.

This _suggests that _up to _90% of the

_cyprinid

genome is maintained

_{quantitatively}

even

though

no

_specific

functions are known for this

DNA. As noted

_previously

_(Gold

and

Price,

1985),

both the

_normality

of distri-butions within

_{cyprinid populations}

and the _apparent constraints on the

_quantity

of DNA which can vary

strongly imply

the

action

of

stabilizing

or

normalizing

se-lection

_{operating through}

the truncation of deleterious extremes

_(Stebbins,

_1966;

Mettler and

_Gregg,

_1969).

_However,

while natural selection may be

influencing

genome size variation within

cyprinid populations,

there is no evidence at _present to indicate that selection favours a

particular cyprinid

species

DNA value relative

to some

_organismal

_parameter

(Gold

and

Amemiya,

1987).

Two

_suggestions

to account for

_interspecies

genome size differences are the

selfish DNA

_hypothesis

_(Doolittle

and

_Sapienza,

_1980;

_Orgel

and

_Crick,

₁₉₈₀₎

and the

_hypothesis

that _{genome size}

_{changes might}

occur

primarily during

speciation

episodes

(Hinegardner,

1976;

Morescalchi, 1977;

Cavalier-Smith,

1978).

The basis for the former is that most

_eukaryotic

_{genomes contain} DNA sequences that

can _{increase in copy} number

_through

differential

replication.

Presumably,

these

sequences are

phenotypically inconsequential,

at least to the

_point

_{where the energy}

expended

in

_replicating

such DNA

_begins

to

_infringe

on the energy needs of the

size data are not inconsistent with the selfish DNA

_hypothesis

in that:

_(i)

there is

significant

variation in genome size within

cyprinid populations

which

_presumably

is

_{phenotypically inconsequential;}

_(ii)

_species

DNA values appear to be more or less

randomly

distributed within the variation which occurs; and

(iii)

individuals at the

high

end of _{the genome size distribution appear to} be removed

_{by negative}

selection.

Alternatively,

one

_might

_predict

that if selfish DNAs contribute

_{significantly}

to

genome size

variation,

the

_underlying

distributions of DNA values should not be normal.

_Species

or

_populations

where selfish DNAs are

_{proliferating}

should show

distributions skewed towards

_higher

_{values; whereas, species}

or

_populations

where

selfish DNAs have accumulated to the

_point

of

_impairing

energy needs should show distributions skewed towards lower values. _{The genome size} distributions in most

cyprinid populations, however,

are

_normal,

and there _{appears to} be no

_general

tendency

towards skewness in either direction.

The

_comparison

of the means and variances of _{genome size} distance between the

cyprinid

genus

Notropis

(species-rich phylad)

versus the centrarchid genus

Lepomis

(species-poor phylad),

suggests that considerable _{genome size}

_change

may occur

during

or be associated with

_{cyprinid speciation episodes.}

Such a

_hypothesis

is

not contradicted

_by

the

_findings

that:

_(i)

genome size variation within

cyprinid

populations

is

_generally

less than _{that among}

_cyprinid

_species;

_(ii)

_cyprinid

_species

genome sizes appear to be

_continuously

and more or less

randomly

distributed

within the variation which occurs; and

(iii)

there are no _apparent associations

between

_species

genome sizes and various

life-history

characteristics

(Gold

and

Price, 1985;

Gold and

_Amemyia,

_1987;

this

_paper).

A

_point

to _note,

_however,

is that the evidence is

_essentially

correlative and it would be difficult to determine

experimentally

whether the correlation was one of cause and effect or one of

association.

_{Moreover, intraspecific}

variation _{in genome size in} both

_cyprinids

and centrarchids can often be as _great as the differences among

species

(Gold

and

Amemyia,

1987;

Ragland

and

_Gold,

_1989;

this

_paper).

This raises some doubt as to the

_strengh

or

validity

of the _apparent correlation between genome size differentiation and

_{speciation since,}

as noted

_{by Ragland}

and Gold

_(1989),

the

generally

lower

_{intraspecific}

variation observed could stem from the

_homogenizing

effects of gene flow within

species.

On the other

_hand,

the

_finding

that ratios of mean _{genome size} distance and

variance in the

Notropis

versus

Lepomis comparison

differ

markedly

from those

reported

for structural genes and

morphology

suggests that different levels of the genome may follow

independent

evolutionary

paths.

The

simplest explanation

for the difference in distance and variance ratios _{is that genome size} evolution is

de-pendent,

in _part, on

_speciation

episodes,

whereas structural gene and

morphological

evolution are

_dependent

_primarily

on

_elapsed

time. This

_explanation

is

unquestion-ably oversimplified

and is based on the

assumptions

that:

_(i)

the models of Avise and

_{Ayala (1975, 1976)}

and Avise

₍₁₉₇₈₎

are

appropriate

and

sufficiently robust,

and

_(ii)

_Notropis

and

_Lepomis

are

_appropriate

taxa for

_comparison.

Neither

as-sumption

is without caveats

_(Avise,

_1977;

_Mayden

_1986),

nor have the models been

tested or used _{in any} other

_organismal

_group outside of

_cyprinid

and centrarchid fishes.

_{Moreover, exactly}

how or

_why

the difference

_might

occur is somewhat

prob-lematic

_given

the

_difficulty

in

_{studying speciation}

in situ

_nascent,

as well as the

pos-sible for any

given

speciation

event. At this

_point,

the conservative thesis is that genome size evolution may be

decoupled

from other _{levels of genome}

_{organization,}

and that genome size may, in

fact,

evolve in a

_{&dquo;quantized&dquo;}

fashion as

_{suggested by}

Cavalier-Smith

_(1985b).

ACKNOWLEDGMENTS

We thank Chris

_Amemiya,

_{Tony Echelle,}

_Gary

_Garrett,

Bill

_Karel,

Mike

_Howell,

Bill Matthews and Bob Stiles for assistance in

_collecting

the

_specimens

used in this

_study,

and we

_gratefully

_acknowledge

the use of facilities at the

_Department

of

Biology

at Samford

_University

in

_{Birmingham during}

our field work in

_Alabama,

and the

_University

of Oklahoma

_Biological

Station on Lake Texoma

_during

our

field work in Oklahoma. We also thank Jim

_Price,

John Bickham and the editors of the

_journal

for constructive comments on the

_manuscript,

and Laura Vilander

for assistance in

_scanning

a few of the

_microscope

slides. The

_scanning

microden-sitometer,

used in the

_research,

was made available

_by

Dr Jim Price of the Soil and

_Crop

Sciences

_Department

at Texas A & M

_University.

The chicken

blood,

used as an internal

standard,

was

_{provided by}

Dr

_{Syed Naqi}

of the Texas A & M

College

of

_Veterinary

Medicine. The work was

_{supported by}

project

H-6703 of the Texas

_Agricultural

_Experiment

Station and

_by

National Science Foundation _grant BSR-8415428. This _{paper represents part} III in the series &dquo;Genome size variation in North American minnows

_{(Cyprinidae).&dquo;}

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