Homospory 2002: An Odyssey of Progress in Pteridophyte Genetics and Evolutionary Biology
Articles T he homosporous vascular plants, including ferns and other graceful, primarily forest-dwelling groups such as the ground pines and horsetails, are the extant representatives of ancient lineages that once dominated the land surface. Although the very well-studied seed plants find their closest relatives (sister group) among these species (Pryer et al. 2001), called homosporous pteridophytes, genetic aspects of ho-mosporous plants have not been studied extensively. In addition to the homosporous species, there are extant het-erosporous pteridophytes—among them the water ferns (Marsileaceae), spike mosses (Selaginellaceae), and quill-worts (Isoetaceae)—and extinct heterosporous lineages, which are well represented in the fossil record. But these " evolutionary experiments " in the heterosporous way of life, whose ancestors are homosporous, did not result in the extraordinary burst of diversification seen among the seed plants, and they are not the focus of this review. However, there are more than 10,000 homosporous pteridophyte species on Earth today, and homospory appears to be the foundation on which all extant plant groups stand. Thus valuable insights for understanding the origin and biodiversity of groups such as the flowering plants could be gained from a solid understanding of homosporous lineages. Unfortunately, developing and testing hypotheses about the genetics and evolution of homosporous pteridophytes has been a difficult enterprise, because although homosporous plants share many structural features with their much-studied heterosporous siblings, the reproductive biology of these two groups is quite different. Nonetheless, the last quarter century has seen considerable change in our understanding of pteridophyte evolution and genetics. This review will consider how new discoveries, insights, and hypotheses have modified our perceptions of homosporous pteridophytes and assess the impact that these changes have had on understanding basic aspects of plant genetics. Homosporous pteridophytes possess biological attributes that complicate a thorough understanding of their genetic systems and evolutionary potential. All vascular plant life cycles feature an alternation of genetically different phases: sporo-phytes, the diploid portion of the life cycle responsible for the production of spores, and gametophytes, the haploid generation that yields gametes. The homosporous pteridophytes produce spores that are uniform in size, whereas heterosporous species (including seed plants) produce spores of two different sizes, microspores and megaspores. This seemingly simple difference in spore size extends to the mode of sexual reproduction. Heterosporous species have unisexual gametophytes—microspores produce exclusively male ga-metophytes and sperm, while megaspores germinate to initiate only female gametophytes and eggs. In contrast, all spores of homosporous species can …