Cytogenetic studies of Poecilia (Pisces)

Natural populations of triploid females resembling the gynogenetic teleost, Poecilia formosa (Girard), occur in northeastern Mexico where they intermingle with diploid populations of this species and the members of congeneric bisexual species such as P. mexicana or P. latipinna. Mitotic configurations from gill epithelial cells show 46 chromosomes for the diploid fishes, but 69 chromosomes for members of the triploid clones associated with P. formosa. Triploid females have erythrocytes that are significantly larger than those from diploid specimens and also show a roughly 50% elevation in the average DNA content of their somatic nuclei. Similar analyses of two functionally incompetent males of P. formosa, of a number of bisexual F₁ and F₂ hybrid offpsring from P. latipinna x P. mexicana, and of females from several other poeciliid species consistently show only diploid DNA levels and somatic chromosome complements where 22N=46. Demonstration of cytogenetic criteria by which females from triploid clones may be clearly distinguished from sympatric diploid specimens of P. formosa or P. mexicana leaves unresolved, for the present, problems of an appropriate systematic designation for natural populations of triploid gynogenetic fishes. The role of sympatric speciation in the evolution of poeciliid genomes is discussed in terms of alternative mechanisms to account for the persistence in nature of a vertebrate triploid of hybrid origin.This work was supported by grants from the National Science Foundation (GB 7393) and from the U.S. Public Health Service (GM 14644).Recipient of a Research Career Development Award from the U.S. Public Health Service (1 K3 GM 3455).