8081

• UCREFRP Catalog Number: 8081
• Author: Buth, D. G., R. W. Murphy and L. Ulmer
• Title: Population Differentiation and Introgressive Hybridization of the Flannelmouth Sucker and of Hatchery and Native Stocks of the Razorback Sucker
• USFW Year: 1987
• USFW - Doc Type: Transactions of the American Fisheries Society
• Copyright Material: YES

108

RUTH ET AL.

TABLE 5.-Comparative measures of presumably in-
trogressed genes in flannel mouth and razorback suckers,
based on relevant loci (Ck-A and M-Icdh-A).

Locality

Flannelmouth sucker

(Theoretical maximum)
Virgin River
Paria River
Little Colorado River
Upper Colorado River

Proportion
of genes
from ftan-
nelmouth
sucker

(1.000)
0.950
1.000
1.000
0.993

Razorback sucker

(Theoretical maximum)
Lake Mohave (adults, biased sample)
Lake Mohave (juveniles. unbiased sample)
Senator Wash Reservoir
Dexler Hatchery

(0.000)
0.02&
0.015
0.000
0.010

are eliminated, even though all were surely ex-
changed in the initial hybridization event. Evi-
dence for introgression in flannelmouth sucker is
found both in the upper Colorado River sample
and in a sample (Virgin River) from the lower
Colorado (Table 5). However, although introgres-
sive hybridization may be an ongoing biological
interaction between these two species, it appar-
ently is rare enough for the species to maintain
the integrity of their respective gene pools. A sim-
ilar si.tuation has recently been reported for the
sympatric common shiner Notropis cornU/us and
striped shiner N. chrysocephalus by Dowling and
Moore (1984). No F, hybrids were identified in
any sample of razorback sucker because these in-
dividuals would be expected to be heterozygous
at all four marker loci: Ck-A, G6pi-Al, M-fcdh-A,
and S-Sod-Al. Introgressed individuals are, at best,
the products ofF2 backcrosses. The question might
be raised as to why the species-specific allelic dis-
tributions at G6pi-Al and S-Sod-Al do not also
supply some evidence of introgressive hybridiza-
tion as do Ck-A and M-fcdh-A. Selection against
the hybrid heterozygous conditions at the former
two loci may be the reason. However, we caution
against overinterpretation of this possibility given
the rarity of Ck-A and M-fcdh-A heterozygotes in
both flannelmouth and razorback suckers; it is
possible that variation of introgressive origin at
G6pi-Al and S-Sod-Al was not observed because
of the relatively small number of specimens ex-
amined.
In summary, although we cannot rule out the
possibility that rare marker alleles were present in

the common ancestor of these species, we believe
the data are in accord with the hypothesis of in-
trogressive hybridization. However, the frequency
of introgressive hybridization is quite low. If the
introgressive interaction between these:two catos-
tomid species is related to habitat medification
rather than to simple variation in the complete-
ness of reproductive isolating mechanisms, future
increased measures of introgression might be ex-
pected as the Colorado River system experiences
continued modification.
The Lake Mohave sample (I a) used in this study
was not randomly selected. Many specimens were
available and those suspected of being hybrids of
razorback and flannel mouth suckers, based on their
morphology, were selected for study. Our allo-
zyme findings thus raise three questions. (I) Is the
morphology of razorback suckers a poor indicator
of introgression? (2) Are introgressed morpholog-
ical and allozyme characters decoupled and dif-
ferentially selected? (3) Is introgression in Lake
Mohave at such a low level that the individuals
selected for study provide the only evidence for
this kind of genetic interaction? The first two ques-
tions can be addressed with some difficulty via
long-term breeding experiments. The more per-
tinent third question is addressed by the allozyme
data from the larger, randomly sampled collection
of juvenile razorback suckers from the same lo-
cality (sample 1 b; Table 5). If introgression, as
measured via allozyme expression, is indeed rare
in Lake Mohave, parental stock could be taken
with a random selection strategy without the need
of electrophoretic screening of such stock.

Evaluation of Current Hatchery Stock

Genes presumed to be introgressed from tlan-
nelmouth suckers are present at very low levels in
razorback suckers in both the hatchery stock and
the samples from Lake Mohave, the source of
hatchery breeding stock (Table 5). The current
hatchery stock has not been appreciably affected
by sampling error in favor of introgressed genes
from tlannelmouth suckers. Indeed, the hatchery
stock has fewer introgressed genes than the pre-
sumed parental stock from Lake Mohave. Thus,
in terms of introgressed genes, the hatchery stock
is at least as "good" as, if not "better" than the
Lake Mohave natural population.
The maintenance of variability in refugium
populations or hatchery stocks has been discussed
recently by Turner (1984) and Meffe (1986). Ge-
netic variability in razorback suckers (Table 3)
might be viewed as low when compared to the