Inbreeding in Natural Populations of the Annual Polyploid Fern Ceratopteris thalictroides (Parkeriaceae)

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Abstract
The degree of gametophytic selfing in natural populations was examined for the annual polyploid fern Ceratopteris thalictroides in several ways. Examination of sexual ontogeny of the gametophytes showed that two types of gametophytes were usually observed: large, cordate hermaphrodites with mature antheridia and archegonia and small, spatulate or amorphous males with numerous antheridia. The occurrence of the former may be correlated with gametophytic selfing, while that of the latter is indicative of the presence of antheridiogen, which can promote gametophytic crossing. Selfing potential was tested by growing isolated gametophytes. In this test, only 3 of 26 spore sources exhibited the presence of genetic load, and even the highest frequency of self-sterile gametophytes in these three was only 3.8%. These gametophytic results predicted a predominantly selfing mating system for this species. This prediction was tested using electrophoretic analysis of allozymes. Polymorphic band patterns were detected for IDH and PGI, and the genetic constitution of these variations was verified through progeny tests. Based on the frequencies of these allelically segregating variants, the rates of gametophytic selfing and F statistics were estimated for a population composed of five subpopulations. The estimated rates of gametophytic selfing were remarkably high, ranging from 0.600 to 1.000 with the mean FIS of 0.886. The estimated FST was 0.109 in IDH and 0.203 in PGI, values suggesting the occurrence of intrapopulational substructuring. The high level of gametophytic selfing in C. thalictroides is probably related to its polyploid nature and annually colonizing habit. An antheridiogen system may operate to compensate for the loss of genetic variability through predominant inbreeding.

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