Abstract
Gamete dynamics theory proposes that anisogamy arises by disruptive selection for gamete numbers versus gamete size and predicts that female/male gamete size (anisogamy ratio) increases with adult size and complexity. Evidence has been that in volvocine green algae, the anisogamy ratio correlates positively with haploid colony size. However, green algae show notable exceptions. We focus on Bryopsidales green algae. While some taxa have a diplontic life cycle in which a diploid adult (=fully grown) stage arises directly from the zygote, many taxa have a haplodiplontic life cycle in which haploid adults develop indirectly: the zygote first develops into a diploid adult (sporophyte) which later undergoes meiosis and releases zoospores, each growing into a haploid adult gametophyte. Our comparative analyses suggest that, as theory predicts: (i) male gametes are minimized, (ii) female gamete sizes vary, probably optimized by number versus survival as zygotes, and (iii) the anisogamy ratio correlates positively with diploid (but not haploid) stage complexity. However, there was no correlation between the anisogamy ratio and diploid adult stage size. Increased environmental severity (water depth) appears to drive increased diploid adult stage complexity and anisogamy ratio: gamete dynamics theory correctly predicts that anisogamy evolves with the (diploid) stage directly provisioned by the zygote.
Highlights
In eukaryote sexual reproduction, a dominant explanation for the evolution of anisogamy
We did not set the male (Vm) and female (Vf ) gamete sizes or zygote size (Vm + Vf ) and the anisogamy ratio (Vf/Vm) as explanatory variables and the response variable, respectively, as the response variable included the explanatory variable. This decision was made because the relationship between an explanatory variable and a response variable cannot be described by a first-degree equation.) we examined the relationship between the volume and complexity of the diploid adults and the anisogamy ratio
The coefficient of variation (CV) of the male gamete volume (CVmale = 0.34) was significantly much smaller than that of the female gamete volume (CVfemale = 3.21) ( p = 0.01; electronic supplementary material, figure S2), and there was no correlation between the male gamete volume and the female gamete volume (PGLS, p = 0.26, adjusted R2 = 0.03)
Summary
A dominant explanation for the evolution of anisogamy Sizes between males and females) from ancestral isogamy (the production of gametes of equal size between 2 mating types) is Parker, Baker and Smith’s model (hereafter, the PBS model) [1]. It relies on gamete competition and has been highly modified from the original [2,3,4,5,6]; PBS merges with gamete limitation into one general model (i.e. gamete dynamics model or GD model) [4,6]. GD predicts increased selection for anisogamy as zygote size increases
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