FARRAR, DONALD R. Department of Botany, Iowa State University, Ames, IA 50011. - Evolution and speciation in Botrychium.
Allozyme studies of Botrychium have revealed several surprising
and seemingly contradictory facts: 1) diploid species have remarkably
low genetic variability, and 2) heterozygous individuals within
populations are extremely rare, but 3) genetic differentiation between
species is high, equaling that between species of most fern genera.
Low genetic variability in species and lack of heterozygotes most
likely result from sexual reproduction via intragametophytic selfing
that is promoted by the underground habitat of the bisexual
gametophytes. If this has always been true of Botrychium, then
how did inbreeding ancestors with low genetic variability generate the
variation to produce the current well-differentiated species. The
expected continuous variation among progeny of such species over time
must have been recently fragmented into precursors of today's species.
Habitat fragmentation by Pleistocene glaciation in the high
latitude/high elevation habitats typical of Botrychium is a
likely cause of recent speciation events. In Botrychium
subgenus Botrychium allotetraploid species outnumber diploids
nearly 2 to 1, and many are widespread and considerably more variable
genetically and morphologically than their diploid parents.
Allotetraploids may be favored because of their greater variability
inherited through multiple origins and enhanced through differential
Key words: allopolyploid, Botrychium, gene silencing, self fertilization, speciation