A New Sexual Form of Pellaea glabella var. glabella from Missouri

Abstract

Between 5% and 10% of fern species are non-sexual. The non-sexual or apogamous type of life cycle is now known in over 80 species among such genera as Adiantum, Asplenium, Cheilanthes, Dryopteris, and Pteris. This form of reproduction bypasses the regular sexual fertilization of egg by sperm, and the details of the pattern were worked out by W. Dopp and Irene Manton in particular (reviewed by Mehra, 1961). The sporophyte and the gametophyte have the same chromosome number. The typical fusion of egg and sperm that occurs in the archegoniuii is lacking. The new fern plant simply grows out of the tissue of the prothallus as a bud. If the chromosome number of the sporopllyte is, for example, 87, the spore will have the same number, the prothallus the same number, and so on. The only change in chromosome number occurs just before spore formation, when there is usually an automatic doubling of chromosome number in the spore mother cells. Instead of producing the normal 16 spore mother cells with 87 chromosomes, the spore-cases produce 8 mother cells with 174. Normal spore formation follows, producing only 32 spores, not the typical 64. If doubling does not occur and there are the usual 16 spore mother cells, the 64 spores which result in a regularly apogamous fern are abortive and will not germinate normally. Thus doubling is required, and the good spores are only those which occur 32 per sporangium. Because of these spore products it is not necessary to make cytological studies to judge in general whether a given fern is apogamous or sexual. By merely looking through the microscope at sporangia from dried herbarium sheets it is