CYTOGENETIC STUDIES OF <i>POECILIA</i> (PISCES). I. CHROMOSOME NUMBERS OF NATURALLY OCCURRING POECILIID SPECIES AND THEIR HYBRIDS FROM EASTERN MEXICO

Abstract

Cytological studies of fish from the subgenus Poecilia (Pisces) including members of an all-female species, P. formosa, have clearly shown diploid, somatic chromosome complements and Robertsonian arm numbers of 46. A triploid somatic chromosome complement of 3N = 69 was demonstrated for members of a naturally occurring variant clone of P. formosa, a form whose general body morphology closely resembles that of its sympatric species P. mexicana. Chromosomes of the different poeciliid species studied are remarkably similar in size, shape and structure, being mostly short acrocentrics, 2 μ long or less.The presence of a chromosome bearing a sub-terminal centromere within the complement of the Amazon molly provides the only direct evidence to date of centric shifts within the chromosome complement of the subgenus Poecilia. This finding also provides the first plausible cytogenetic basis for concluding that P. formosa is of relatively recent origin. Comparisons of relative chromosome lengths from both mitotic and meiotic configurations of related species indicate that the heteromorphic chromosome pair of P. formosa probably was derived by hybridization between P. latipinna and P. mexicana. Finally, as suggested by the numbers and forms of chromosomes in atypical, triploid specimens of P. formosa, their complements apparently contain both a maternal diploid set of chromosomes from the all-female species, and a paternal haploid set of chromosomes from P. mexicana.