Evol Ecol Res 11: 57-77 (2009)     Full PDF if your library subscribes.

Alternation of haploid and diploid generations: evolution by gamete amplification

Priya Iyer and Joan Roughgarden

Department of Biological Sciences, Stanford University, Stanford, California, USA

Correspondence: P. Iyer, Department of Biological Sciences, Stanford University, Herrin 424 Serra Mall, Stanford, CA 94305, USA.
e-mail: priyai@stanford.edu


Question: What selects for the alternation of haploid and diploid generations in algal taxa?

Mathematical methods: We derive the growth rates of haplontic, diplontic, and haplo-diplontic populations as functions of ploidy-dependent survival probabilities and propagule production rates. We use a population genetic model with a single locus coding for haplonty, diplonty, and haplo-diplonty to obtain the evolutionarily stable conditions for the fixation of each of the three ploidy-cycles. We simulate the evolutionary dynamics to demonstrate the convergence to these equilibria.

Key assumptions: Non-overlapping generations, ploidy-dependent propagule production and survival rates, and the synchronous release of gametes into water by the entire population. We assume that the fertilization probability of eggs increases as a function of the sperm density encountered.

Conclusions: The ploidy-cycle selected is predicted to be the one with the highest population growth rate defined by propagule production and survival rates. Haplo-diplonty may be selected in low fertilization environments because it mitigates the problem of sperm limitation, as the gamete concentration amplifies over subsequent generations. Diplonty may be favoured in more variable environments because diploidy confers higher viability in such environments.

Keywords: algae, alternation of generations, fertilization probability, haplo-diplonty, ploidy, sperm limitation.

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