AOBPreview originally published online on October 6, 2009
Annals of Botany 2009 104(7):1353-1361; doi:10.1093/aob/mcp245
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Developmental morphology of strap-shaped gametophytes of Colysis decurrens: a new look at meristem development and function in fern gametophytes
Department of Chemical and Biological Sciences, Japan Women's University, 2-8-1 Mejirodai, Tokyo 112-8681, Japan
* For correspondence. E-mail ryoko{at}fc.jwu.ac.jp
Received: 12 May 2009 Returned for revision: 13 July 2009 Accepted: 26 August 2009 Published electronically: 6 October 2009
Background and Aims: The gametophytes of most homosporous ferns are cordate–thalloid in shape. Some are strap- or ribbon-shaped and have been assumed to have evolved from terrestrial cordate shapes as an adaptation to epiphytic habitats. The aim of the present study was to clarify the morphological evolution of the strap-shaped gametophyte of microsoroids (Polypodiaceae) by precise analysis of their development.
Methods: Spores of Colysis decurrens collected in Kagoshima, Japan, were cultured and observed microscopically. Epi-illuminated micrographs of growing gametophytes were captured every 24 h, allowing analysis of the cell lineage of meristems. Light microscopy of resin-sections and scanning electron microscopy were also used.
Key Results: Contrary to previous assumptions that strap-shaped Colysis gametophytes have no organized meristem, three different types of meristems are formed during development: (1) apical-cell based – responsible for early growth; (2) marginal – further growth, including gametophyte branching; and (3) multicellular – formation of cushions with archegonia. The cushion is two or three layers thick and intermittent. The apical-cell and multicellular meristems are similar to those of cordate gametophytes of other ferns, but the marginal meristem is unique to the strap-shaped gametophyte of this fern.
Conclusions: The strap-shaped gametophytes of C. decurrens may have evolved from ancestors with a cordate shape by insertion of the marginal meristem phase between the first apical-cell-based meristem and subsequent multicellular meristem phases. Repeated retrieval of the marginal meristem at the multicellular meristem phase would result in indefinite prolongation of gametophyte growth, an ecological adaptation to epiphytic habitats.
Key words: Apical cell, apical meristem, Colysis decurrens, cushion, development, evolution, ferns, gametophytes, gametophyte adaptation, marginal meristem
Present address: Midorimachi Junior High school in Chiba, 2-3-1 Midorimachi, Chiba 263-0023, Japan.