title: Coleochaete and the origin of sporophytes.
abstract: •
  
  
  OBJECTIVE
  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.•
  
  
  METHODS
  Simple optimization models are developed of the number of zygotes per maternal thallus and number of zoospores per zygote.•
  
  
  RESULTS
  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.•
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, Massachusetts 02138 USA.
authors.forename: David
authors.initials: D
authors.lastname: Haig
mesh_terms: D005075:Biological Evolution; D002455:Cell Division; D057946:Charophyceae / Q000254:growth & development*; D008954:Models, Biological; D011003:Ploidies; D012098:Reproduction
keywords: Coleochaete; alternation of generations; matrotrophy; size vs. number; sporophyte