Diversity, development and evolution of archegonia in land plants

Abstract

Abstract We review the diversity and development of archegonia, the female reproductive organs of land-plant gametophytes. The archegonium is a uniquely land-plant structure, and studies of its evolution benefit from use of a comparative approach in a phylogenetic context. Archegonia of most land plants share a common developmental motif, here termed a T-shaped pattern. A primary axial cell produces a primary cover cell and a central cell by horizontal division. The upper cell usually divides vertically and the lower one horizontally. In mosses such as Atrichum, the T-shaped stage is shifted towards the end of archegonium development, whereas in vascular plants it appears at the beginning of development, but these stages are still probably homologous. The fully exposed archegonia are traditionally viewed as an ancestral (plesiomorphic) condition in land plants, but there is no direct support for this view. We speculate that the fully exposed condition is derived and synapomorphic for setaphytes (mosses and liverworts). The fully sunken hornwort archegonia may be similar to the ancestral type of land-plant archegonia. Developmental evidence suggests that archegonium necks of setaphytes and tracheophytes are not homologous to each other. The neck wall of pteridophytes is composed of four-celled tiers, and one such tier is present in gymnosperms with motile male gametes. Neck-cell arrangement is much more plastic in archegonia of gymnosperms with sperm cell delivery by pollen tube (siphonogamy), in which the neck plays a role similar to pollen-tube transmitting tissue of angiosperms. Angiosperm synergids are probably homologues of gymnosperm neck cells, and the angiosperm egg cell is probably homologous to the ventral canal cell of gymnosperms. Developmental genetic bases of archegonium diversity in land plants remain to be understood. Even descriptive developmental data are currently missing or controversial for some key lineages of land plants.