The Population Structure of an Asexual Vertebrate, Poeciliopsis 2 Monacha- lucida (Pisces: Poeciliidae)

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Abstract
Genotypic diversity in populations of asexually reproducing species in homogeneous environments must necessarily be a balance between new clones entering the population via mutation, de novo synthesis via hyridization, immigration and aberrant recombinational events, and clonal extinction. Population size, then, should affect clonal diversity because this parameter affects both the expected number of mutants that enter the population and the rate of random extinction: the smaller the population, the fewer clones. Poeciliopsis 2 monacha-lucida is an allfemale, ovoviviparous fish that is truly asexual in a genetic sense. This species inhabits numerous bedrock pools in the headwaters of the Rio Fuerte and other drainages in Sonora and Sinaloa, Mexico. The subject of this study is a system of demes in several headwater tributaries of the Arroyo Jaguari branch of the Rio Fuerte and a single deme in the Arroyo de Guirocoba branch. The genetic structure of these populations was studied by tissue grafting; the role of migration and population size in the determination of population structure was studied by mark-and-recapture experiments. The Arroyo Jaguari and Arroyo Guirocoba branches are isolated from one another, but the occurrence of histocompatible fish in distinct tributaries of the Jaguari indicates that migration is a significant determinant of population structure within that branch. Serially numbered fish released prior to the rainy season and collected after the rainy season indicate that Poeciliopsis can overcome substantial barriers to migrate upstream as well as down. The P. 2 monacha-lucida population in the Jaguari is not large; individual demes typically consist of 100 juvenile and adult females and the entire Jaguari complex supports only about 3,000 juveniles and adults. Despite their diminutive sizes, populations are surprisingly rich in histocompatibility clones. A sample of 24 females resolved into six or seven distinct clones: one from the Guirocoba (3 specimens) and six from the Jaguari (21 specimens). The Shannon statistic for histocompatibility clonal diversity is 0.740. A separate study of 23 enzymatic and muscle protein loci in 155 specimens detected only two clones (electromorph clones I and II, Vrijenhoek and Leslie, 1977). Electromorph clone I corresponds to a single histocompatibility clone, but electromorph clone II is a composite of at least five histocompatibility clones. Unidirectional histocompatibility is a feature of the clonal structure of P. 2 monacha-lucida; e.g., tissue grafted from genetic line C to B takes whereas tissue grafted from B to C rejects. This indicates that evolution of histocompatibility clones has occurred in P. 2 monacha-lucida. Poeciliopsis 2 monacha-lucida is like another asexual (gynogenetic) fish, Poecilia formosa, in that both have diverse clonal structures. These two form a sharp contrast, however, to several species of Cnemidophorus (whiptail lizards). Single clones have exclusive domain over large geographical areas in the latter, and one species, C. neomexicanus, may be comprised of a single histocompatibility clone.

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