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106 BUTH IT AL. TABLE 2.- Estimates of genetic variability in four samples of flannelmouth sucker and three samples of razorback sucker. Calculations are based on the gene products of 21 presumptive loci listed in Table I. Mean number of Proportion Mean proportion of Mean proportion of alleles per locus of loci heterozygotes heterozygotes Locality (SE) polymorphic observed' (SE) expecte~b (SE) Flannelmouth sucker Virgin River 1.1 (0.1) 0.095 Om5 (0.027) 0.033 (0.025) Paria River 1.1 (0.1) 0.143 0.048 (0.026) 0.048 (0.026) Lillie Colorado River 1.0 (0.0) 0.048 0.024 (0.024) 0.023 (0.023) Upper Colorado River 1.5 (0.1) 0.429 0.027 (0.013) 0.027 (0.013) Razorback sucker Lake Mohave (sample la) 1.4 (0.1) 0.333 0.034 (0.0 I 5) 0.036 (0.016) Senator Wash Reservoir 1.2 (0.1) 0.143 0.024 (0.0 I 3) 0.028 (0.017) Dexter hatchery 1.4 (0.1) 0.286 0.018 (0.007) 0.022 (0.010) , Direct count of heterozygotes. b Unbiased estimate (Nei 1978) based on Hardy-Weinberg expectations. loci, Acp-A. Adh-A. S-lcdh-Al, and Pgm-A, agree- ment with equilibrium conditions occurred. How- ever, these are "trivial" situations because the polymorphism was based, in each case, on a single heterozygous individual resolved in the entire set of samples. The other five loci, Ck-A. G-3-pdh-A, G6pi-A I, G6pi-BI. and G6pi-Bl, exhibited far greater departures from equilibrium conditions, significantly so for the latter four loci (P < 0.05). Thus, these data reveal statistically significant structuring of gene flow patterns within the range of flannel mouth sucker. While no complete allelic differences were found between samples (indica- tive of complete restriction of gene flow), differ- ences'in allele frequency, specifically the geo- graphic limitations of certain polymorphisms, yielded the significant levels of gene flow restric- tion. For comparative purposes, geographic differ- entiation was also quantified by Nei's (1978) coef- ficients of unbiased genetic identity (I) and un- biased genetic distance (D) for all pairwise combinations of populations (Table 3). Wright's (1965) F-statistics (Table 4) were calculated to TABLE 3.-Nei's (1978) genetic identity coefficients (/: above the diagonal of dashes) and genetic distance coef- ficients (D: below diagonal) between pairs of samples of flannelmouth sucker. Calculations are based on the gene products of 21 presumptive loci (Table I). Lillie Upper Virgin Paria Colorado Colorado Locality River River River River Virgin River 0.992 0.976 0.982 Paria River 0.008 0.994 1.000 Lillie Colorado River 0.024 0.007 0.998 Upper Colorado River 0.018 0.000 0.002 quantify departure from Hardy-Weinberg pro- portions within samples (Fls) and in the total sam- ple (FIT), and to measure the genetic differentia- tion among samples (FH). All polymorphic loci contributed to a negative mean value for FIS, in- dicative ofa moderate amount of intra sample het- erozygote excess (minimal inbreeding, ifany). The positive mean value for FIT denotes a moderate amount of heterozygote deficiency in the total sample. An FST value of 0.231 is associated with a "great" deal of differentiation among the sam- ples (sensu Hartl 1981). TABLE 4.-Summary of F-statistics across all intra- specifically variable loci for four geographic samples of flannel mouth sucker and two native samples (la and 2) of razorback sucker. Fls = within samples; FIT = total sample; FST = among samples. Locus Fls FIT FST Flannelmouth sucker Acp-A -0.013 -0.003 0.010 Adh-A -0.013 -0.003 0.010 Ck.A -0.099 -0.029 0.064 G6pi-AI -0.139 0.289 0.376 G6pi-B' -0.184 -0.047 0.116 G6pi-Bl -0.156 -0.052 0.090 G3pdh-A -0.028 0.187 0.209 S-Icdh-Al -0.013 -0.003 0.010 Pgm-A -0.013 -0.003 0.010 Mean -0.108 0.148 0.231 Razorback sucker Adh-A -0.029 -0.014 0.014 Ck-A -0.029 -0.014 0.014 G6pi-Al -0.059 -0.029 0.029 G6pi-Bl -0.043 -0.021 0.021 M-Icdh-A -0.029 -0.0/4 0.014 S-Mdh-A I 0.189 0.189 0.000 S-Mdh-Al -0.077 -0.037 0.037 Pgm-A -0.049 -0.040 0.009 Mean 0.061 0.072 0.011