There has been increasing interest in recent years in the reproductive biology of ferns and much attention has been paid to the gametophytes. As is well-known, one of the traits that fern gametophytes have evolved is the ability for sexual reproduction. In sexually reproducing plants, the evolutionary importance of a mechanism that ensures outbreeding to preserve genetic diversity needs no emphasis. However, most homosporous ferns produce bisexual gametophytes that promote inbreeding and attainment of homozygosity in the progeny. Adaptations that prevent these deleterious features in the gametophyte facilitate genetic variability in the population and successful colonization of the species. In this chapter we will review some of the strategies in the reproductive biology of fern gametophytes that have contributed to genetic diversity and evolutionary potential of the species. A word of caution is necessary before we proceed further. Most of what we know in this area concerns homosporous ferns, those represented in nature by their green, thalloid gametophytes. Heterosporous ferns, as well as homosporous ferns with subterranean gametophytes have barely been studied.

The blight of homozygosity

To begin with, some comments about the breeding system of ferns are in order. Insofar as functional antheridia and archegonia are born on different gametophytes, heterosporous ferns are considered to be obligatorily heterothallic (self-incompatible, thus requiring two compatible gametophytes for sexual reproduction).