Barriga-Sosa, I.*; Arredondo-Figueroa, J. L.: Morphological and allozyme variation in Chirostoma grandocule Steindachner (Pisces:Atherinopsidae) from Lake P�tzcuaro, M�xico.

We investigated the levels of population differentiation of the silverside fish species Chirostoma.grandocule Steindachner in Lake P�tzcuaro based on morphometric and allozyme data and relate such information to aspects of life history and changes in their natural habitat. We analysed eighteen morphometric characters, eight meristic and eleven putative enzyme-coding loci from C. grandocule samples of two different localities (“subpopulations”) within the lake and of three different years (Janitzio 1996, 1999 and Ichupio 1997). Principal Component (PC) analysis with the morphometric data indicate the separation of two groups (grouped into four components, 66.78%). Among the variables that contributed the highest variation were: pelvic fin length of base (PfbL), head length (HL), eye diameter (ED), dorsal 2nd lengh (D2L) and snout length (SnL). Discriminant analysis (DA) with morphometric data also indicated separation between samples (87.10% correctly classified) Janitzio and Ichupio (Wilk’s l= 0.13-48, P< 0.000). Allozyme data resolved seven polymorphic loci at P.99 level. C. grandocule showed levels of He that ranged from 0.0134 to 0.0319 (average HS = 0.0234). Genetic differences were observed between years and between “subpopulations”, with six alleles of Janitzio never detected in Ichupio and one of Ichupio not present in both years of Janitzio samples. Genetic distances (DR & co-ancestry) between samples ranged from 0.0154 to 0.0341. Weir & Cockerham’s F-statistics provided evidence of significant heterogeneity among “subpopulations” and years (jack-knifed qS = 0.0252, S.D. 0.0126). Our morphometric and allozyme results are consistent and suggest that Chirostoma grandocule is encountering an actual process of population differentiation in lake P�tzcuaro.