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.' 438 AMMERMAN AND MORIZOT TABLE 2.-Allele frequencies at nine polymorphic loci in four populations of Colorado squawfish. Frequency Dexter 1981 Dexter 1987 Colorado River Green River Locus Allele (N= 7) (N= 31) (N= 26) (N = 26) EST-I* *100 0.929 0.950 0.577' 0.577b *95 0.071 0.050 0.423 0.365 *91 0.0 0.0 0.0 0.058 bGAL* *253 0.357 0.242 0.346 0.365 *100 0.643 0.758 0.654 0.635 GPI-I* *122 0.071 0.452 0.365 0.250 *100 0.929 0.548 0.635 0.750 GPI-2* *379 0.0 0.194 0.038' 0.0 *100 1.0 0.758 0.942 1.0 *-195 0.0 0.048 0.019 0.0 LDH.J* *-29 0.071 0.355 0.160 0.0 *- 100 0.929 0.645 0.840 1.0 MDH-2* *103 0.071 0.016 0.077 0.115 *100 0.929 0.984 0.923 0.885 PEPS* *100 0.929 1.0 1.0 1.0 *62 0.071 0.0 0.0 0.0 TPI-I* *102 0.0 0.0 0.0 0.019 *100 1.0 1.0 1.0 0.981 TPI-2* *100 1.0 1.0 0.962 1.0 *-1041 0.0 0.0 0.038 0.0 , Significantly different from expected Hardy-Weinberg proportions at P = 0.0 I. b Significantly different from expected Hardy-Weinberg proportions at P = 0.05. populations were fixed for the most common al- lele at the GPI-2* locus. In both Dexter 1987 and the Colorado River populations, the most cath- odal and most anodal allozymes of GPI-2 were seen in very low frequency. Rare alleles may not be present simply because of small sample sizes. At EST-I *, the only population that expressed a third allele was the Green River population. Both hatchery stocks had the EST- J * 100 allele in higher frequency (0.929 and 0.950) than the two wild populations (0.577), which suggested a trend to- ward fixation of the most common allele. Seven other polymorphic loci were diallelic. Variability at three of these loci was restricted to a single population. A Green River specimen ex- pressed the single varian tat T P I-I *; this exem- pifies cryptic variability as discussed by Aquadro and A vise (1982). Triose-phosphate isomerase-l allozymes from brain plus eye tissue were variable on Ridgway buffer, whereas on tris-EDT A-borate and tris-citrate buffers, the locus appeared to be monomorphic. Our results demonstrated how some alleles would often go undetected in labo- ratories where single-buffer systems are used to measure variability. Variation at TPI-2* was pres- ent only in the Colorado River samples. The single peptidase variant (PEPS*62) with leucyl-glycyl- glycine as substrate was in the Dexter 1981 pop- ulation. Although established with individuals from the Colorado and Green rivers, the Dexter 1981 hatchery stock has gained an allele not de- tected, but probably present in low frequencies, in natural populations. Four other loci (bGAL *, GPI- 1*, MDH-2*, LDH-3*) exhibited allelic variation in two or more populations. Mean heterozygosity values ranged from 2.60,. (Dexter 1981) to 5.3% (Colorado River; Table 3). In comparison to nine other cyprinid genera, whose average heterozygosity was 3.78 ::t 0.80% (Avise and Ayala 1976), these values for Colorado squawfish are not surprising. Previous biochemi- cal work on the Sacramento squawfish Ptychochei- Ius grandis yielded a much lower heterozygosity value of 1.1 % (Avise and Ayala 1976). Similarly, the proportion of polymorphic loci in our study ranged from 9.1 to 13.6%, whereas Avise and Ayala (1976) found somewhat lower variability in the Sacramento squawfish, which had only 4.2% polymorphic loci. The high heterozygosity (4.7%) in the Dexter 1987 hatchery population may reflect high values of heterozygosity in individuals from the Yampa River population that were used as breeding stock. Yampa River populations, represented by Dexter 1987 stock, have a heterozygosity level within the range of the two other wild populations examined