Showing papers by "Leo W. Beukeboom published in 1999"
TL;DR: There appears to be weak niche differentiation between sexuals and parthenogenetic individuals in Lago di Caldonazzo in late summer, a consequence of low dispersal between local populations and stochastic effects within them.
Abstract: Theoretical models on the costs and benefits of sexual reproduction usually assume that sexual and parthenogenetic individuals coexist and are identical, except for their mode of reproduction. Empirical studies, however, show that conspecific sexuals and parthenogens can differ in ecological preferences and geographical distribution, which complicates the investigation of the costs and benefits of sex. The freshwater planarian Schmidtea polychroa exists in a sexual and a sperm-dependent, parthenogenetic form. The latter produce fertile sperm and mate, but received sperm is used only to induce parthenogenetic embryo development. We compared the spatial and ecological distribution between forms within a lake from which both had been reported. Forty samples showed large differences in the relative frequencies of sexuals and parthenogens. Nineteen samples contained both biotypes. All but one of the 13 ecological parameters that we measured, could not explain a significant part of the variance in relative abundance of each type. Only leech abundance had a significant, negative effect on the presence of sexual individuals. The causes of this effect remained unclear. We also estimated the amount of genetic isolation between sites and between reproductive modes, using body coloration as a genetic marker. Large differences were found between sites, suggesting isolation of local populations by migration barriers. There were smaller differences between sexuals and parthenogens within sites, suggesting that genetic exchange between biotypes may be limited. We conclude that there appears to be weak niche differentiation between sexuals and parthenogens in Lago di Caldonazzo in late summer. Fluctuations in relative frequency appears to be a consequence of low dispersal between local populations and stochastic effects within them.
18 citations
TL;DR: It is argued that the pluralistic approach (West et al., 1999) could be further strengthened by not concentrating solely on population-level processes, but by encompassing the important role that individuals may play.
Abstract: West et al. (1999) convincingly argue that combining traditional hypotheses on the maintenance of sex into a pluralistic framework provides a more plausible explanation for the enigmatic success of sexuality. By merging (1) more ef®cient elimination of deleterious mutations with (2) better tracking of environmental changes (often parasites) and allowing for synergism between both, West et al. (1999) show that sexuality becomes much more robust against invasion by asexuality. Their approach abandons traditional attempts to ®nd a single and suf®cient explanation for sex. However, once accepting that a mixture of ingredients may be the best recipe to explain sex, we strongly suggest adding at least one more component. Here, we argue that the pluralistic approach (West et al., 1999) could be further strengthened by not concentrating solely on population-level processes, but by encompassing the important role that individuals may play. We focus on two assumptions that population geneticists often make and that are inherent to the hypotheses within the pluralistic framework, namely that offspring are produced (1) randomly and (2) without paternal care. Under these assumptions, offspring produced by a sexual female can be represented as a quality array of randomly produced progeny in a 1:1 sex ratio (Fig. 1, bold lines). For asexuals, this distribution is compressed to a single all-female class with some small variance due to mutation (not shown). Sex is favoured when the advantage of producing few, better adapted and less mutation-loaded offspring outweighs the cost of producing males plus the cost of producing low-®tness offspring. Under random mating, high-quality sexual individuals lose most as they are likely to have relatively poorer mates, whereas low-quality individuals will bene®t as they are likely to have better mates. This equalizing effect limits the bene®ts of sex. However sex is not usually random. Sexual individuals can actively in ̄uence the quality of their progeny (Fig. 1, dashed lines) and data from behavioural ecology suggest that they do this speci®cally in an attempt to capitalize on the bene®ts and reduce the costs of recombination. This results in a net advantage of
11 citations