The Evolution and Maintenance of Monoecy and Dioecy in Sagittaria Latifolia (Alismataceae)

Abstract

Plant species rarely exhibit both monoecious and dioecious sexual systems. This limits opportunities to investigate the consequences of combined versus separate sex function on mating patterns and genetic variation and the analysis of factors responsible for the evolution and maintenance of the two sexual systems. Populations of the North American clonal aquatic Sagittaria latifolia are usually either monoecious or dioecious and often grow in close geographic proximity. We investigated mating patterns, genetic structure, and relationships between the two sexual systems using allozyme variation in populations from southern Ontario, Canada. As predicted, selfing rates in monoecious populations (n = 6, mean = 0.41) were significantly higher than in dioecious populations (n = 6, mean = 0.11). Moreover, marker-based estimates of inbreeding depression (delta) indicated strong selection against inbred offspring in both monoecious (mean delta = 0.83) and dioecious (mean delta = 0.84) populations. However, the difference in selfing rate between the sexual systems was not reflected in contrasting levels of genetic variation. Our surveys of 12 loci in 15 monoecious and 11 dioecious populations revealed no significant differences in the proportion of polymorphic loci (P), number of alleles per locus (A), or observed and expected heterozygosity (H(o) and H(e), respectively). Strong inbreeding depression favoring survival of outcrossed offspring may act to maintain similar levels of diversity between monoecious and dioecious populations. Despite geographical overlap between the two sexual systems in southern Ontario, a dendrogram of genetic relationships indicated two distinct clusters of populations largely corresponding to monoecious and dioecious populations. Reproductive isolation between monoecious and dioecious populations appears to be governed, in part, by observed differences in habitat and life history. We suggest that selfing and inbreeding depression in monoecious populations are important in the transition from monoecy to dioecy and that the maintenance of distinct sexual systems in S. latifolia is governed by interactions between ecology, life history, and mating.

Implications of Extreme Life Span in Clonal Organisms: Millenary Clones in Meadows of the Threatened Seagrass Posidonia oceanica

“…Genotypic richness was estimated from the number of ramets sampled (N) and the number of multilocus genotypes detected (G), as suggested by Dorken et al [61]: …”

Sexual dimorphism in leaf nitrogen content but not photosynthetic rates in Sagittaria latifolia (Alismataceae)

“…The species is unusual in having populations with two contrasting (and genetically determined) sexual systems: dioecy and monoecy (Dorken et al 2002;Dorken and Barrett 2004a). Our focus here is on a dioecious population near the northern shore of Lake Erie.…”

Molecular phylogenetics of Hydatellaceae (Nymphaeales): Sexual‐system homoplasy and a new sectional classification

“…Although less common than the transition from hermaphroditism to dioecy, cosexuality has been inferred to be a potentially derived state in several angiosperm lineages ( Sytsma et al, 1991 ;Dorken et al, 2002 ;Obbard et al, 2006 ;Case et al, staminate vs. pistillate RUs. Reduction in pollen production in some species may also be a simple function of a high selfi ng rate (e.g., Ornduff, 1969 ;Sicard and Lenhard, 2011 ).…”

“…The correlates of dioecy and causes of the evolution of dioecy from cosexuality have garnered much theoretical attention, with the change usually considered to occur via gynodioecy or monoecy (e.g., Charnov, 1982 ;Thomson and Brunet, 1990 ;Renner and Ricklefs, 1995 ;Charlesworth, 1999 ;Sakai and Weller, 1999 ;Barrett, 2002Barrett, , 2010Vamosi et al, 2003 ). The reverse process, the evolution of cosexuality from dioecy, has received far less theoretical attention (e.g., Wolf and Takebayashi, 2004 ;Delph, 2009 ), and there are fewer purported transitions ( Sytsma et al, 1991 ;Dorken et al, 2002 ;Obbard et al, 2006 ;Case et al, 2008 ;Volz and Renner, 2008 ;Schaefer and Renner, 2010 ). The direction(s) and frequency of sexual-system change in Hydatellaceae are unclear.…”