8081 - Laserfiche WebLink

Home Browse Search

INTROGRESSION IN COLORADO RIVER CATOSTOMIDS Razorback sucker. - Eleven of 2 I loci were polymorphic in at least one of the samples of ra- zorback sucker (Table J). In tests for conformance with Hardy-Weinberg equilibrium expectations, all but one sample lacked significance (P > 0.05); the genotype array at the S-Mdh-A! locus in the Dexter Hatchery sample exhibited a significant departure (P < 0.00 I) from equilibrium expec- tations. This deviation was the result of a defi- ciency of heterozygotes and an excess in both ho- mozygote categories. Both of the other samples (la and 2) contained this diallelic polymorphism at this locus but both yielded genotype arrays con- forming to equilibrium expectations. Measures of genetic variability were comparable among the samples of razorback suckers (Table 2). The contingency chi-square analysis of samples la and 2 revealed agreement with equilibrium conditions for all polymorphic loci. No genetic differentiation was calculated by Nei's (1978) method between the two native samples of razor- back sucker and between the hatchery sample and either of these native samples; i.e., 1= 1.000, and D = 0.000. Positive mean F-statistics for the na- tive samples (Table 4) indicated a small degree of overall heterozygote deficiency within and be- tween samples (mostly due to S-Mdh-A'). An FST value of 0.011 calculated between the nalive sam- ples is considered to reflect "little" differentiation (sensu Hartl 1981). Interspecific Differentiation Divergence between species is often marked by complete allelic differentiation at one or more loci (A vise 1976). These "marker" loci can be ex- tremely useful in studies of biochemical identifi- cation and hybridization. Four loci, Ck-A. G6pi- A!. M-Icdh-A. and S-Sod-A!. exhibited allelic distributions that were species-specific, or nearly so, and could be used to separate flannelmouth suckers from razorback suckers. In addition, many population-specific minor alleles at other loci served to distinguish these taxa (Table I). The average unbiased genetic distance and genetic identity coefficients (Nei 1978) computed between flannelmouth sucker and razorback sucker were D = 0.220 and 1= 0.802, respectively. The primary interest of this study was to ex- amine flannelmouth and razorback suckers for dif- ferences in gene expression as well as for differ- ences in allelic composition at homologous loci. The catostomid fishes are ancestrally tetraploid (Uyeno and Smith J 972) and would, in theory, be expected to express duplications (beyond the dip- 107 loid condition) at all loci (Ohno 1970) whose products can be resolved via electrophoretic methods. However, with multiple (duplicated) loci, one is free to vary (to acquire mutatio~s that might have been selected against in the single-locus con- dition and may even be "silenced" or "turned off" in a regulatory sense) as long as the other locus remains functional (Ohno 1970). Thus, ancestral- ly tetraploid catostomid species can differ in the number of genes controlling given enzyme sys- tems (e.g., Ferris and Whitt 1977). Such differ- ences have been used in phylogenetic studies (Fer- ris and Whitt 1978b; Buth 1979, 1980). However, the number of genes controlling the enzyme sys- tems examined proved to be identical for razor- back and flannel mouth suckers, eliminating this character as one useful for the identification of hybrids. Assessment of Possible Introgression Allozymic evidence for introgressive hybridiza- tion between flannefmouth suckers and razorback suckers can be found in the examination of the distribution of the gene products of those marker loci previously discussed. Two of these loci, Ck-A and M-/cdh-A. have allelic distributions suggest- ing some gene flow between the two species. In- traspecific variation involving creatine kinase (Ck- A) is rare (Ferris and Whitt 1978a) and the reso- lution of Ck-A heterozygosity in both species in this study would not be expected. Both alleles are present in both species and the predominant allele in each species (e.g., Ck-A[aJ in the flannel mouth sucker) is present in low frequency in the other species. There is, of course, the possibility that both alleles at both loci were present in the com- mon ancestor of these catostomids and that they have drifted differentially or been selected for, al- though not to fixation, in these two species. The hypothesis of the sharing of these alleles as prim- itive traits can only be addressed through the ex- amination of additional, closely related, species of Catostomus (see Murphy et al. 1984). The situa- tion involving M-Icdh-A is somewhat similar and is summarized in Table I. If introgressive hybridization is accepted as the means to account for the sharing of the other species' alleles at Ck-A and M-/cdh-A. one can observe that gene flow is proceeding both from flannelmouth to razorback sucker (with M-/cdh-A favored) and from razorback to flannelmouth sucker (with Ck-A favored). Here, "favored" means that an introgessed allele remains in the new gene pool while other "nonfavored" alleles