Towards a synthesis of evolutionary modes and mechanisms in homosporous pteridophytes

Abstract

Recent advances in understanding the genetics and evolution of homosporous pteridophytes require re-examination of the theories concerning their phylogeny and population biology. Contradictions in homosporous pteridophyte biology include isozymic diploidy despite high chromosome numbers, outcrossing in diploids despite hermaphroditic gametophytes, inbreeding in allopolyploids despite outcrossing in their progenitors, and low genetic identities between congeners despite substantial morphological similarity. These facts are consistent with several testable hypotheses about the history of homosporous species. The isozymic diploidy/chromosomal polyploidy paradox could have resulted either from repeated cycles of allopolyploidy and gene silencing or from high chromosome numbers in pteridophyte ancestors. Genetic load in diploids may drive outcrossing whereas homoeologous chromosomes in allopolyploids may buffer against this load and permit inbreeding. Low congeneric genetic identities may be linked to the antiquity of the lineages, the isolation of species via accumulation of many genetic differences, and may also indicate that there are numerous, unrecognized, cryptic species. Speciation may occur through primary divergence between diploid population (orthospeciation), secondary interactions between species to initiate polyploid species (nothospeciation) and/or tertiary speciation via reciprocal gene silencing in polyploids (metaspeciation). Testing these hypotheses through molecular chromosomal, and populational studies may generate new directions for research on homosporous pteridophytes