DISTRIBUTION OF SEXUAL AND APOMICTIC POPULATIONS OF ANTENNARIA PARLINII.

Abstract

The evolution and adaptive value of sexual reproduction have been the subject of extensive theoretical exploration (Williams, 1975; Maynard Smith, 1978). The major problem centers about the cost of as Williams expresses it. If both sexual and asexual means of reproduction are available to an individual well adapted to its environment, why should it break up its complex adaptive system of genes by segregation at meiosis, on the chance that equally adaptive complexes will be reconstituted by fertilization? Any empirical answer to this question will require comparisons of related populations that reproduce sexually and/or asexually. The presence over a long period of time of both sexuality and asexuality in closely related populations would indicate that both modes of reproduction are equally adaptive. If reproductive mode varies with geography or ecological coiiditions, sexuality can be assumed to confer an adaptive advantage under some conditions and asexuality under others, or else the change in relative proportions of the two systems occurs over very long periods of time. Dioecious, agamospermous species of plants may be useful for testing hypotheses about sexuality because apomictic females will produce only female offspring. Consequently, the sex ratio within a population is a valuable though not an infallible clue to the incidence of asexuality. Both sexual dioecism and agamic seed production exist in Antennaria (Asteraceae, Inuleae), a genus of perennial herbs found in cool temperate to arctic regions of the northern hemisphere. Taxonomists investigating North American Antennaria during the 1880's and 1890's observed that many widespread species consist of populations containing only pistillate plants. The presence of apomixis via diplospory (Gustafsson, 1944; Stebbins, 1950) was determined for species of eastern North America by Stebbins (1932). We believe that three entities recognized by Fernald (1945), namely A. fallax Greene, A. parlinii Fern., and A. munda Fern., should be combined into a single variable species, A. parlinii s.l. (Bayer and Stebbins, in press). Antennaria parlinii is chiefly hexaploid (2n = 84), but includes a few tetraploid (2n = 56) and octoploid (2n = 112) populations (Bayer and Stebbins, 1981). Populations of A. parlinii may consist entirely of pistillate plants in addition to others in which staminate plants occur (Fernald, 1936). Herbarium specimens from Ohio at The Ohio State University show that staminate plants have been much more commonly collected in the southeastern (unglaciated) as opposed to the northwestern (glaciated) part of Ohio. Consequently, this state appeared to be particularly favorable for comparing sexual, apomictic and mixed populations of A. parlinii. We have determined that the great majority of both kinds of populations have the same chromosome number, 2n = 84. The objectives of the present study are to describe the distribution of apomictic and sexual populations in Ohio and their correlation with the glacial history, to document the nature of facultatively apomictic populations in the glacial