Incipient Speciation in Freshwater Fish Species from Two Isolated Watersheds

The process of speciation occurs as a result of restricted gene flow between segments of an interbreeding population occupying different geographic areas. This separation may result in isolated populations which undergo genetic and phenotypic changes. The Wisconsin glacial period, which ended approximately 17,500 years ago, dramatically altered the geography of North America. The glacier covered almost the entire North America as it advanced. Areas that were not covered with ice provided suitable habitats (refugia) for relict species that were previously widespread in the northern section of the continent. As the ice sheet retreated, animals and plants were able to return to the once glaciated areas. However, as the glacier retreated, it disrupted the distribution patterns of aquatic animals and produced large numbers of small isolated populations. As a result of the Wisconsin glaciation, the Bronx and Saw Mill Rivers now exist in separate but parallel watersheds. The main focus of this research was to decipher if there is evidence of incipient speciation between freshwater fish populations that reside in two isolated river systems: the Bronx and Saw Mill Rivers that have been separated since the Wisconsin glaciation approximately 17,500 years ago. Both rivers contain many of the same fish species. The target species were Rhinichthys atratulus (Blacknose Dace), Etheostoma olmstedi (Tessellated Darter) and Catostomus commersonii (White Sucker). In this study the number of chromosomes and their morphology, the morphological, meristic and osteological differentiation, and the genetic divergence of these fish populations from both rivers were investigated. The result of the chromosomal analyses indicated that for each fish species from both rivers there was no difference between populations. Three metaphase spreads were obtained for each of the 22 Blacknose Dace for a total of 66 spreads examined. The karyotype consisted of 16 metacentric, 28 submetacentric, 2 subtelocentric, and 4 telocentric, yielding a diploid number of 2n=50 chromosomes. The diploid number for the Tessellated Darter was 2n = 48. A total of ten fish from the Bronx River and twelve from the Saw Mill River was examined. Their karyotype consisted of 42 acrocentric and 6 telocentric chromosomes. The White Sucker had a diploid number 2n = 98 with 5 metacentric, 7 submetacentric and 86 subtelocentric chromosomes. Unlike the results for the chromosomal component of this study, morphometric, meristic, osteologic and genetic differences were observed between the different species from both rivers. An analysis of variance (ANOVA) was conducted on all morphometric, meristic, and osteologic characters. Morphometric analyses indicated that the differences were greater in the Blacknose Dace than in the Tessellated Darter and the White Sucker. Of the fifty-two morphometric characters examined for the Blacknose Dace and the White Sucker, twenty-seven characters (52 %) for the Blacknose Dace and seventeen (32%) for the White Sucker were significantly different (p \u3c 0.05). In addition, only fifteen of sixty-two characters (24%) for the Tessellated Darter were significantly different (p \u3c 0.05). Of the fifteen meristic counts conducted only 4 (26%) was significantly different (p \u3c 0.05) for the Blacknose Dace and for the White Sucker. There were no significant differences observed for the meristic count for the Tessellated Darter between the two rivers. The p-values for the osteological data were comparable to that of the meristic data as there was a significant difference between the populations for the Blacknose Dace (25 %) and the White Sucker (12 %), but not for the Tessellated Darter. Molecular data supported the morphometric, meristic and osteologic analyses in that there are genetic variations between the different populations of fish from both rivers. Using mtDNA control region for the Blacknose Dace and the White Sucker and cytochrome b gene for the Tessellated Darter, a similar pattern of divergence was observed between the populations from the Bronx and Saw Mill Rivers. Each fish species had haplotypes common to both rivers. However, there were haplotypes not shared between the two rivers. Significant differences in haplotype frequencies within the three fish species indicate the populations are beginning to diverge (P \u3c 0.0001). For this reason, due to genetic drift, genetic differences could lead to incipient speciation and eventually result in actual speciation if the present environmental conditions persist within the Bronx and Saw Mill Rivers