Can Parthenogenetic Cnemidophorus laredoensis (Teiidae) Coexist with Its Bisexual Congeners?

Abstract

-Wright and Lowe (1968) and Cuellar (1977) suggested that parthenogenetic Cnemidophorus can survive only in isolation from their bisexual congeners because the latter would otherwise eliminate the parthenoforms through competition and/or hybridization. Data from studies of the parthenogenetic clone LAR-A of the Cnemidophorus laredoensis subgroup suggest this hypothesis is not correct. The bisexual species C. gularis occurs at 24 of the 39 sites at which LAR-A has been found, and neither form is consistently more abundant than the other at syntopic sites. The literature documents many cases where parthenogenetic whiptails coexist with their bisexual congeners, suggesting parthenoforms do not require isolation as previously believed. One of two things must be happening to allow parthenoforms to coexist with bisexuals: (1) competition and do not occur; or (2) these processes do occur but some other factor counteracts their effect. At present, too little data exist to evaluate which of these alternatives is correct. However, analysis of lizard diets at three sites of syntopy and data reported in studies of other parthenoforms are used to advance testable hypotheses in the hope of stimulating future research. The lizard genus Cnemidophorus (Teiidae) includes several forms that reproduce by true parthenogenesis in addition to many bisexual species. The parthenoforms originate as hybrids between bisexual species; the hybrids then reproduce asexually to generate clones of genetically identical individuals (Wright, 1978; Cole, 1979). The existence of parthenoforms presents biologists with an interesting evolutionary problem: how do such forms persist in a world dominated by sexually reproducing species that are presumably better adapted to the environment through natural selection? In their classic paper, Wright and Lowe (1968) proposed the weed hypothesis, which states that parthenogenetic whiptails can become established only in disturbed, marginal, or ecotonal habitats because such habitats are unsuitable for bisexual whiptails. A slightly different view was proposed by Cuellar (1977) who suggested parthenogenetic whiptails occupy only floodplain, beach, or lakeside habitats because parthenoforms can rapidly colonize areas where natural disasters periodically eliminate lizard populations, and because bisexual species are usually absent from such habitats. The implication of both hypotheses is that parthenoforms are poor competitors, presumably because their limited genotypic diversity translates into narrow ecological breadth (Bell, 1982; Case and Taper, 1986). Therefore parthenoforms can avoid competitive exclusion by their bisexual congeners only by occupying disturbed habitats. Cuellar (1977) also emphasized that isolation from bisexual forms ensures that a parthenoform will not lose its genetic identity through hybridization, a phenomenon termed destabilizing hybridization by Lynch (1984). This occurs if diploid parthenoforms mate with males of bisexual species to produce triploid hybrids that are either sterile or are parthenogenetic themselves (Lynch, 1984; Vrijenhoek, 1989). The latter case should lead to the replacement of diploid parthenoforms by triploid parthenoforms. Hybridization with bisexual species might also be detrimental to a parthenoform if the fertilization of a parthenogenetic egg caused the egg to fail to develop, resulting in waste of reproductive effort (White and Contreras, 1979). Both Wright and Lowe (1968) and Cuellar (1977) suggested that parthenoforms should rarely be found at site where bisexual species occur, and in those ra e instances where the two forms do co-occur, the bisexual species should be rare enough that it does not exert a strong impact on the parthenoform. However, these hypotheses have not been tested rigorously. The parthenogenetic Cnemidophorus laredoensis subgroup comprises several allodiploid clones that occupy a narrow zone on either side of the Rio Grande from Del Rio (Val Verde County), Texas, to within 16 km of the Gulf of Mexico (Walker, 1987a, b). The subject of this paper is the clone LAR-A, (originally described as Cnemidophorus laredoensis by McKinney et al., 1973). A second major clone, named LAR-B, discovThis content downloaded from 207.46.13.160 on Mon, 17 Oct 2016 05:18:30 UTC All use subject to http://about.jstor.org/terms M. A. PAULISSEN ET AL. TABLE 1. Snout-vent lengths (SVL) and mass of LAR-A and C. gularis at three sites in Texas. Values are mean + standard deviation, sample sizes are given in parentheses. Statistically significant differences, as evaluated by Mann-Whitney U tests, are indicated by an asterisk. The sample sizes for SVL and mass at Garceno differ because lizards were not weighed during the 1 May collection.