A comparison of three replication strategies in complex multicellular organisms: Asexual replication, sexual replication with identical gametes, and sexual replication with distinct sperm and egg gametes

This paper studies the mutation-selection balance in three simplified replication models. The first model considers a population of organisms replicating via the production of asexual spores. The second model considers a sexually replicating population that produces identical gametes. The third model considers a sexually replicating population that produces distinct sperm and egg gametes. All models assume diploid organisms whose genomes consist of two chromosomes, each of which is taken to be functional if equal to some master sequence, and defective otherwise. In the asexual population, the asexual diploid spores develop directly into adult organisms. In the sexual populations, the haploid gametes enter a haploid pool, where they may fuse with other haploids. The resulting immature diploid organisms then proceed to develop into mature organisms. Based on an analysis of all three models, we find that, as organism size increases, a sexually replicating population can only outcompete an asexually replicating population if the adult organisms produce distinct sperm and egg gametes. A sexual replication strategy that is based on the production of large numbers of sperm cells to fertilize a small number of eggs is found to be necessary in order to maintain a sufficiently low cost for sex for the strategy to be selected for over a purely asexual strategy. We discuss the usefulness of this model in understanding the evolution and maintenance of sexual replication as the preferred replication strategy in complex, multicellular organisms.

💡 Research Summary

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The paper investigates the evolutionary competition between asexual and sexual reproduction by constructing three stylized population‑genetic models. All models assume diploid organisms whose genomes consist of two chromosomes; each chromosome is either functional (identical to a master sequence) or defective. A mutation rate μ governs the conversion of a functional chromosome to a defective one during replication, and a selection coefficient s reduces the fitness of individuals carrying defective chromosomes. The three replication strategies are: (1) asexual spore production, (2) sexual reproduction with identical gametes, and (3) sexual reproduction with distinct sperm and egg gametes.

In the asexual model, each adult directly produces diploid spores that immediately develop into new adults. Because spore formation and maturation occur without an intervening haploid stage, mutation and selection act simultaneously on the same generation. The mean fitness of the asexual population can be expressed as
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