THEEVOLUTION OFALTERNATIVEGENETICSYSTEMS ININSECTS

Abstract

There are three major classes of insect genetic systems: those with diploid males (diplodiploidy), those with effectively haploid males (haplodiploidy), and those without males (thelytoky). Mixed systems, involving cyclic or facultative switching between thelytoky and either of the other systems, also occur. I present a classification of the genetic systems of insects and estimate the number of evolutionary transitions between them that have occurred. Obligate thelytoky has arisen from each of the other systems, and there is evidence that over 900 such origins have occurred. The number of origins of facultative thelytoky and the number of reversions from obligate thelytoky to facultative and cyclic thelytoky are difficult to estimate. The other transitions are few in number: five origins of cyclic thelytoky, eight origins of obligate haplodiploidy (including paternal genome elimination), the strange case of Micromalthus, and the two reversions from haplodiploidy to diplodiploidy in scale insects. Available evidence tends to support W.D. Hamilton's hypothesis that maternally transmitted endosymbionts have been involved in the origins of haplodiploidy. Bizarre systems of extrazygotic inheritance in Sternorrhyncha are not easily accommodated into any existing classification of genetic systems.

Transitioning from egg to embryo: Triggers and mechanisms of egg activation

“…Such strategies range from obligate sexual reproduction in which both males and females participate in the formation of a new organism, to parthenogenetic reproduction (thelytoky) in which females produce females without genetic contribution from males. Between these two extremes there is facultative parthenogenesis, conditional parthenogenesis, and occasional or sporadic parthenogenesis (reviewed in Normark, 2003). The classic example of facultative parthenogenesis occurs in the order Hymenoptera (bees, wasps, ants, and so on), to release Ca 2ϩ from the endoplasmic reticulum (ER).…”

Absence asymmetry: The evolution of monorchid beetles (Insecta: Coleoptera: Carabidae)

“…In the case of hermaphrodites, optimal resource partitioning (Angeloni et al, 2002) between different sex gonads could be the selective pressure that drives testes to a unitary structure. Very few hexapods are structural hermaphrodites, but the condition is known in Plecoptera, Isoptera, Blattodea, and Hemiptera (Rościszewska and Soldá n, 1999;Normark, 2003). However, the presence of viable gametes of both sexes in a single individual is only reported from a few species of Hemiptera (HughesSchrader, 1963;Hughes-Schrader and Monohan, 1966).…”

The evolution of sex ratios and sex-determining systems

“…Males develop from unfertilized eggs and are haploid, whereas females develop from fertilized eggs and are diploid. Haplodiploidy has evolved multiple times and occurs in all Hymenoptera (ants, bees, wasps and sawflies) and several other invertebrate taxa, including beetles, thrips, scale insects and mites [53].…”

“…Sex ratio selection has therefore been the traditional explanation for the evolution of haplodiploidy [25]; however, recent evidence suggests that genomic conflict is a more probable explanation [56]. Haplodiploidy has arisen in organisms with highly gregarious kin groups [53,56] that, in addition to favoring biased sex ratios, often creates conflicts of interest between maternally and paternally derived alleles within the offspring ( [56,57] see also Box 2). Most notably, haplodiploidy will select for paternal alleles that favor fitness of individual offspring despite compromising the fitness of the mother and the brood as a whole.…”