Abstract
• Zygotes of Coleochaete are provisioned by the maternal thallus before undergoing 3-5 rounds of division to produce 8-32 zoospores. An understanding of the selective forces favoring postzygotic divisions would be relevant not only to the interpretation of Coleochaete life history but also to the origin of a multicellular diploid phase in embryophytes.• Simple optimization models are developed of the number of zygotes per maternal thallus and number of zoospores per zygote.• Zygotic mitosis is favored once zygotic size exceeds a threshold, but natural selection usually promotes investment in additional zygotes before zygotes reach this threshold. Factors that favor production of fewer, larger zygotes include multiple paternity, low fecundity, and accessory costs of zygote production. Such factors can result in zygotes exceeding the size at which zygotic mitosis becomes profitable.• Coleochaete may possess large zygotes that undergo multiple fission because of accessory costs associated with matrotrophy, including costs of cortical cells and unfertilized oogonia. The unpredictability of fertilization on land is proposed to have increased accessory costs from unfertilized ova and, as a consequence, to have favored the production of larger zygotes that underwent postzygotic division to produce diploid sporophytes.
Highlights
Coleochaete may possess large zygotes that undergo multiple fission because of accessory costs associated with matrotrophy
Coleochaete fell from favor in these debates after Allen (1905, 1906) concluded that the first two divisions of its zygospore were the heterotypic and homotypic divisions
This paper presents simple life-history models of the transition from a singlecelled zygote to a multicelled ‘fruit.’ These models are agnostic about the precise nature of Coleochaete’s postzygotic divisions whether meiotic, mitotic, or something else
Summary
2. Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge MA 02138 USA. 1. Manuscript received _______; revision accepted ______. Premise of study: Zygotes of Coleochaete are provisioned by the maternal thallus before undergoing 3–5 rounds of division to produce 8–32 zoospores. An understanding of the selective forces favoring post-zygotic divisions would be relevant to the life history of Coleochaete but to the origin of a multicellular diploid phase in embryophytes
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