Seed plant gametophytes: Still the forgotten generation?!?
Friedman, William E. , Williams, Joseph H. .
Modularity and the evolution of the female gametophyte in ancient angiosperms.
For over a century, the monosporic seven-celled/eight-nucleate Polygonum-type female gametophyte has been viewed as plesiomorphic among angiosperms. In Polygonum-type female gametophytes, two haploid female nuclei are incorporated into the central cell, and fusion of a sperm cell with the binucleate central cell produces a triploid endosperm with a complement of two maternal genomes and one paternal genome. Recently, we have shown that members of two ancient angiosperm lineages, Nymphaeales and Austrobaileyales, produce four-celled female gametophytes with an egg cell, two synergids and a uninucleate central cell. We conclude that the angiosperm female gametophyte is constructed of modular developmental subunits, each of which is characterized by a common developmental pattern: 1) positioning of a single nucleus within a cytoplasmic domain (pole) of the female gametophyte; 2) two free-nuclear mitoses to yield four nuclei within that domain; and 3) partitioning of three uninucleate cells adjacent to the pole such that the fourth nucleus is confined to the central region of the female gametophyte (central cell). Within Nymphaeales and Austrobaileyales, female gametophytes are characterized by a single developmental module that produces a four-celled/four-nucleate structure with a haploid uninucleate central cell. A second pattern, typical of Amborella and the overwhelming majority of eumagnoliids, monocots and eudicots, involves the early establishment of two developmental modules that produce a seven-celled/eight-nucleate female gametophyte with two haploid nuclei in the central cell. Our evolutionary developmental analysis indicates that the first angiosperm female gametophytes were composed of a single (four-celled) module, which upon double fertilization yielded a diploid endosperm. Early in angiosperm history, ectopic expression of this basic developmental module resulted in the production of two developmental modules within the female gametophyte and the formation of a seven-celled/eight-nucleate structure that yields a triploid endosperm with the 2:1 maternal to paternal genome ratio characteristic of most flowering plants.
1 - University of Tennessee, Department of Botany, 437 Hesler Biology, Knoxville, Tennessee, 37996, USA
2 - University of Colorado, Department of Ecology and Evolutionary Biology, UCB 334, Boulder, Colorado, 80309-0334, USA
Presentation Type: Symposium
Location: Ballroom 1 (Cliff Lodge)
Date: Monday, August 2nd, 2004
Time: 10:15 AM