The Absence of Genetic Load in a Morphologically Variable Sexual Species, Ceratopteris thalictroides (Parkeriaceae)

Abstract

Genetic and mating system analysis of Ceratopteris thalictroides indicates that recessive deleterious and lethal genes (genetic load) are rare or non-existent in most sporophytes studied, although nearly all plants exhibit high probabilities of an outbreeding reproductive system in gametophyte populations. Genetic load expres- sion is shown to be directly correlated with three complementary parameters of gametophyte morphology (gametangia sequence of hermaphroditic prothalli, ratio of male to hermaphrodites, and number dehisced antheridia in the two gametophyte types). Population studies indicate that most populations are genetically depauperate for lethal genes and that there is a direct relationship between population size and percent of sporophytes with deleterious genes. The low level of genetic load in most populations is hypothesized to be due to their mode of establishment, age, and frequency of vegetative reproduction. Ceratopteris forms a complex array of species which are unique among homosporous ferns in several features. They are aquatic, annual, some- what weedy, reproduce profusely vegetatively from sporophytic buds (Lloyd, 1973, 1974a), and produce a gametophyte generation which ex- hibits rapid development and sexual maturation, and chemical antheri- dogens. In addition, recent work by Klekowski (1973a), Klekowski and Hickok (1974), and Hickok (1977a, 1978) indicates that homoeologous pairing during meiosis occurs and can produce an array of spore ge- notypes even from genetically homozygous individuals. As presently circumscribed, the genus is composed of four broadly defined species, three diploids, C. cornuta (Palisot de Beauvois) Le Prieur, C. pteridoides (W. J. Hooker) Hieronymus, C. richardii Brongniart, and one tetraploid/hexaploid, C. thalictroides (Linnaeus) Brongniart (Lloyd, 1974a). Hickok (1977b) has found that the diploid species are actively