Genetic load and soft selection in ferns

Abstract

Genetic load data for two homosporous ferns are summarized and the mean frequency of recessive sporophytic lethals per zygote calculated. Fifty-one spore collections of Onoclea sensibilis had a mean of 0·587 lethals per zygote, and 129 spore collections of Osmunda regalis had a mean of 2·39 lethals per zygote. Each spore collection represents a meiotic sample from a single individual (genet). Both species form hermaphroditic gametophytes with extensive capacities for simple polyembryony. Simple polyembryony results in a form of soft selection in panmictic populations and thus higher equilibrium frequencies for lethals are possible than in organisms without simple polyembryony. When ferns are inbred, the high load levels are expressed. Load levels in ferns may reflect a balance between soft selection in panmictic populations and hard selection under conditions of inbreeding. The sheltering effect of simple poly-embryony is demonstrated for both mutational and heterotic load.