Abstract
Ciliate mating systems are highly diversified, providing unique opportunities to study sexual differentiation and its implications for mating dynamics. Many species of ciliates have multiple (>2) sexes. More sexes may mean more choice and an opportunity for evolution of preferential mating. We asked if the multiple sexes of the ciliate Tetrahymena thermophila mate preferentially among each other. We quantified pairing frequencies among four sexes of T. thermophila using experiments that allowed the sexes to compete as mating partners. We found that all sexes mated equally frequently among each other, that is, we found no evidence of preferential mating with respect to sex. This suggests that the “mate choice” in this ciliate is binary, between whether to form a pair or not and, in this regard, sex facilitates only self-/non-self-distinction. Thus, presence of multiple sexes does not necessarily result in the evolution of mating bias, which could decrease the maximum amount of mating that would otherwise be possible in a population. Our result of random mating verifies a key assumption in the theoretical model of sex ratio evolution in T. thermophila. Investigation into molecular differences between the sexes will be necessary to reveal the mechanistic basis of random mating among them.
Highlights
Mating is random when two individuals in a population are just as likely as any other two individuals to mate
Unicellular species have more than two sexes, raising an obvious yet previously unanswered question: do more sexes mean more choice, thereby making evolution of mate preference among the sexes inevitable? In other words, is selective mating observed when there is an opportunity to choose between many compatible sexes? For instance, the multiple sexes of a species could form a hierarchy from the best phenotype to the least preferred one
Our aim was to investigate whether preferential mating occurs among sexes, when more than two sexes are present in a population
Summary
Mating is random when two individuals in a population are just as likely as any other two individuals to mate. Dynamics of nonrandom mating have been studied in sexually dimorphic species in which size, sound, and color often describe the most preferred phenotype [2]. Mate-preference has been demonstrated in the yeast Saccharomyces cerevisiae, in which the highest amount of pheromone produced defines the most preferred phenotype for the cells of either sex [3]. It is largely unclear how mates are chosen in other unicellular organisms. The multiple sexes could be grouped such that sexes within a group mate more frequently with each other than those between groups, resulting in pronounced mating preferences between groups
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