文摘
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Summary
Nearly all animals reproduce sexually through the production and fusion of sperm and egg cells, yet little is known about the ancestry of animal sexual reproduction. Moreover, the sexual cycle of the closest living relatives of animals, the choanoflagellates [], remains completely unknown. The choanoflagellate Monosiga brevicollis possesses a 鈥渕eiotic toolkit鈥?of genes [], but the lack of polymorphisms detected during genome sequencing precluded inferences about its ploidy or sexual cycle []. Here, we report that a related choanoflagellate, Salpingoeca rosetta [], has a sexual life cycle and transitions between haploid and diploid states. Haploid cultures of S.聽rosetta became diploid in response to nutrient limitation. This ploidy shift coincided with anisogamous mating, during which small flagellated cells fused with larger flagellated cells. Distributions of polymorphisms in laboratory strains of S.聽rosetta provided independent evidence of historical recombination and mating. The ability of S.聽rosetta to produce morphologically differentiated gametes and to engage in sexual reproduction has implications for both reconstructing the evolution of sex in the progenitors of animals and establishing classical genetics in choanoflagellates.
