evolution and maintenance of monoecy and dioecy in Sagittaria latifolia (Alismataceae).
Published: 2003
Language: eng
Number of pages: 236
Description: The evolution of separate sexes (dioecy) from combined sexes (hermaphroditism) is a central problem in evolutionary biology. Determining the conditions under which unisexual individuals replace individuals with two sexual pathways to reproductive fitness has attracted considerable theoretical interest. However, the use of experimental approaches to address this problem has been limited by the overall rarity of plant species comprised of both dioecious and hermaphroditic populations. Here, I investigate experimentally the selective mechanisms responsible for the evolution of dioecy in the aquatic herb Sagittaria latifolia  (Alismataceae) and also provide the historical, geographic and ecological context for this evolutionary transition. In S. latifolia both monoecious (hermaphrodites with unisexual flowers) and dioecious populations occur, often in close geographic proximity. This variation in sexual strategy not only raises questions about the origin of dioecy, but also about how the contrasting sexual systems are maintained.Theoretical models indicate that selection for dioecy in S. latifolia  should be favoured in monoecious populations with substantial selfing and strong inbreeding depression. Empirical estimates of these parameters indicate that in some monoecious populations the conditions for the evolution of dioecy (s x delta > 0.5) are indeed satisfied. Extensive clonality restricting outcross pollen dispersal by the generalist pollinators of S. latifolia is likely to be the principal cause of high geitonogamous selfing rates in monoecious populations. Distinct sexual systems are maintained in S. latifolia because monoecious and dioecious populations possess contrasting life-history traits adapted to wetland habitats differing in disturbance and competition, respectively. Thus, ecological selection maintains reproductive isolation of the two sexual systems in S. latifolia .Using variation in chloroplast DNA from 76 populations distributed across eastern North America, I found that dioecy likely evolved from monoecy in the southeastern U.S.A. with populations of both sexual systems migrating northwards following deglaciation. An analysis of variation at allozyme loci in populations from southern Ontario indicated that restricted gene flow promotes the maintenance of the two sexual systems in areas of geographic overlap. Formal genetic analysis revealed that dioecy in S. latifolia is governed by the segregation of alleles at two putatively linked loci with males heterozygous at both female- and male-sterility loci. In monoecious populations, individuals are homozygous for sex-determining alleles, but there is also quantitative genetic variation for sex allocation. Dioecy probably evolved via the gynodioecy pathway in S. latifolia and involved genes of both large and small phenotypic effects.
Notes: Adviser:  Spencer C. H. Barrett.

Thesis (Ph.D.)--University of Toronto, 2003.

Electronic version licensed for access by U. of T. users.

Source: Dissertation Abstracts International, Volume: 65-05, Section: B, page: 2177.