What do isogamous organisms teach us about sex and the two sexes?

Abstract

One contribution of 15 to a theme issue 'Weird sex: the underappreciated diversity of sexual reproduction'. Isogamy is a reproductive system where all gametes are morphologically similar, especially in terms of size. Its importance goes beyond specific cases: to this day non-anisogamous systems are common outside of multicellular animals and plants, they can be found in all eukaryotic super-groups, and anisogamous organisms appear to have isogamous ancestors. Furthermore, because maleness is synonymous with the production of small gametes, an explanation for the initial origin of males and females is synonymous with understanding the transition from isogamy to anisogamy. As we show here, this transition may also be crucial for understanding why sex itself remains common even in taxa with high costs of male production (the twofold cost of sex). The transition to anisogamy implies the origin of male and female sexes, kickstarts the subsequent evolution of sex roles, and has a major impact on the costliness of sexual reproduction. Finally, we combine some of the consequences of isogamy and anisogamy in a thought experiment on the maintenance of sexual reproduction. We ask what happens if there is a less than twofold benefit to sex (not an unlikely scenario as large short-term benefits have proved difficult to find), and argue that this could lead to a situation where lineages that evolve anisogamy-and thus the highest costs of sex-end up being associated with constraints that make invasion by asexual reproduction unlikely (the 'anisogamy gateway' hypothesis).This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'.

The fitness effects of delayed switching to sex in a facultatively asexual insect

“…as female age at mating increased (see Table S2), which suggests that females that mate later in life may be more likely to produce unfertil- Our finding that a solely parthenogenetic strategy conferred lower fitness than a solely sexual one is consistent with previous studies on other facultative systems. In many facultatively parthenogenetic insects, parthenogenesis results in lower reproductive performance compared to sex, manifesting as depressed fecundity (Chang et al, 2014), poor offspring viability , and/or reduced offspring lifespan (Kramer & Templeton, 2001 suggestions that developmental, genetic, ecological or evolutionary constraints could prevent parthenogenesis evolving from sexual ancestors (Burke & Bonduriansky, 2017a;Engelstadter, 2008;Lehtonen, Kokko, & Parker, 2016;Neiman, 2004;Vrijenhoek, 1989). For example, several preadaptations at the cellular level have been identified as essential for optimal parthenogenetic development (Engelstadter, 2008), and mechanisms that maintain genetic heterozygosity between generations are thought to be crucial for long-term success of parthenogens (Simon, Delmotte, Rispe, & Crease, 2003).…”

Invasion and extinction dynamics of mating types under facultative sexual reproduction

“…In isogamous species, the gamete size differentiation that defines the sexes in anisogamous species is absent. Instead, these species produce gametes that are morphologically simi-lar (Lehtonen et al, 2016). Despite this physical similarity, the gametes of isogamous species are typically not interchangeable, but rather fall into one of a number of genetically determined self-incompatible gamete classes, termed mating types.…”

Evolution of asymmetric gamete signaling and suppressed recombination at the mating type locus

“…Binding of 72 ligand and receptor originating from the same cell can obstruct this interaction. To capture this, we 73 define the strength of the incoming signal for cell 1 when it interacts with cell 2 as,…”