Genetical Features of Ferns as Contrasted to Seed Plants

Abstract

pects. As Figure 1 shows, an angiosperm seed is a propagule which develops after meiosis and syngamy have occurred. This propagule contains the results of double fertilization; endosperm tissue and an embryo. Upon seed germination this embryo will develop into a mature sporophyte. The genotype of this sporophyte does not reflect its current isolation but rather is related to the breeding system of the parental sporophyte population. Thus it may be homozygous or very heterozygous. In contrast, an homosporous fern spore is a propagule that develops after meiosis but prior to syngamy. In this case only a single haploid cell is dispersed and upon germination undergoes a series of mitotic cell divisions leading to the development of a haploid, generally autotrophic plant, the gametophyte. This organism is functionally hermaphroditic, simultaneously developing male and female gametangia at some point during its life. Self-fertilization (intragametophytic selfing) results in a single diploid cell, the zygote, which is completely homozygous. This homozygous cell undergoes mitotic cell divisions leading to the development of an embryo and eventually a mature sporopbyte. Thus the establishment of a sporophyte from a single propagule in an homosporous fern results in a completely homozygous individual, whereas in a seed plant the sporophyte genotype may be heterozygous. Because of the above genetic distinctions between a seed and a spore, the evolutionary future of disjunct populations of homosporous ferns and angiosperms might predictably be very different. The angiosperm sporophyte, if it is perennial and dioecious, may live long enough to encounter a sporophyte of the opposite sex, outcross, and form progeny. These resulting progeny in all likelihood would be