Breeding Systems and Seed Size in a Neotropical Flora: Testing Evolutionary Hypotheses

Abstract

A well-known, but largely untested, prediction in plant reproductive ecology is that dioecious taxa should produce larger, more, higher-quality, or better-defended seeds than cosexual taxa. Using a data set composed of 972 species in 104 families, representing the flora of the Tambopata Wildlife Reserve (Madre de Dios, Peru), we evaluated the first component of this prediction, examining ecological and evolutionary relationships between breeding system and mean seed size with two kinds of tests. First, we conducted cross-species analyses to determine whether species with different breeding systems differed significantly with respect to mean individual seed size. Second, we used a hypothesized phylogeny to identify pairs of the most closely related taxa or clades within the Tambopata community that differed with respect to breeding system. Comparing pair members allowed us to determine whether evolutionary divergence in breeding system (between taxa with unisexual vs. cosexual individuals) was consistently associated with evolutionary change in seed size. In both analyses, we controlled for potentially confounding effects of growth form by examining these relationships within woody and nonwoody taxa. Cross-species analyses revealed that dioecious species produced larger seeds than cosexual species among woody species, shrubs, lianas (each growth form analyzed separately), and all species pooled, but not among trees. Phylogenetically independent contrasts upheld the significant association between breeding system and seed size among woody taxa, lianas, and all taxa pooled, but not among shrubs. We discuss the implications of our findings for evolutionary hypotheses regarding associations between dioecy and seed size.

The role of breeding system in community dynamics: Growth and mortality in forests of different successional stages

“…In contrast, most cosexual individuals, experiencing inbreeding depression, are less competitive at every stage of their life history (Charlesworth & Charlesworth, 1978a, 1978b; Sakai, Karoly, & Weller, 1989). This competitive advantage may possibly contribute to the coexistence of dioecious plants and other breeding systems in the absence of fecundity and population density advantages in primary forest communities (Vamosi, Mazer, & Cornejo, 2008; Queenborough et al., 2009; Yunyun Wang et al. unpublished).…”

Surviving in a Cosexual World: A Cost-Benefit Analysis of Dioecy in Tropical Trees

“…This rarity may reflect demographic costs associated with dioecy (Bawa 1980): when a large proportion of the population are males, population growth is reduced-all else being equal-relative to when all individuals produce seeds. Having fewer seedproducing individuals is thus expected to constitute a substantial fitness disadvantage (Queenborough et al 2007;Vamosi et al 2007Vamosi et al , 2008. Nonetheless, dioecy has evolved independently at least 100 times (Charlesworth 2002;Barrett 2010) and is represented among almost half of angiosperm families (Renner and Ricklefs 1995).…”

“…Although increased genetic variation and decreased inbreeding depression may partly explain the benefits, it is difficult to explain dioecy simply as a mechanism favoring outbreeding given that most hermaphroditic species also have efficient outbreeding mechanisms (Bawa 1974;Renner and Ricklefs 1995;Freeman et al 1997). Identification of the benefits of dioecy and quantification of how dioecious species compensate for the costs is therefore an intriguing and long-standing challenge in ecology (e.g., Darwin 1877; Opler and Bawa 1978;Armstrong and Irvine 1989;Renner and Ricklefs 1995;Freeman et al 1997;Vamosi 2008;Queenborough et al 2009). …”

“…Dioecious species may be able to invest more in reproduction, growth, and/or survival because they produce only staminate or pistillate flowers (Bawa 1980;Queenborough et al 2007Queenborough et al , 2009Vamosi et al 2008). Reproductive investment focused on one flower type may enable increased seed production, an earlier age of reproduction, more frequent reproduction, and/or production of better-quality seeds, leading to higher seedling establishment or survival (Heilbuth et al 2001;Queenborough et al 2009).…”

“…Reproductive investment focused on one flower type may enable increased seed production, an earlier age of reproduction, more frequent reproduction, and/or production of better-quality seeds, leading to higher seedling establishment or survival (Heilbuth et al 2001;Queenborough et al 2009). However, the two studies that have compared these vital rates between dioecious and nondioecious species have found little evidence that females in dioecious species benefit from the absence of costs associated with the production of staminate flowers (Vamosi et al 2008;Queenborough et al 2009).…”