5.6. Sperm storage, fertilization, and sex determination
Many female insects store the sperm that they receive from one or more males in their sperm storage organ, or spermatheca. Females of most insect orders have a single spermatheca but some flies are notable in having more, often two or three. Sometimes sperm remain viable in the spermatheca for a considerable time, even three or more years in the case of honey bees. During storage, secretions from the female’s spermathecal gland maintain the viability of sperm.
Eggs are fertilized as they pass down the median oviduct and vagina. The sperm enter the egg via one or more micropyles, which are narrow canals that pass through the eggshell. The micropyle or micropylar area is orientated towards the opening of the spermatheca during egg passage, facilitating sperm entry. In many insects, the release of sperm from the spermatheca appears to be controlled very precisely in timing and number. In queen honey bees as few as 20 sperm per egg may be released, suggesting extraordinary economy of use.
The fertilized eggs of most insects give rise to both males and females, with the sex dependent upon specific determining mechanisms, which are predominantly genetic. Most insects are diploid, i.e. having one set of chromosomes from each parent. The most common mechanism is for sex of the offspring to be determined by the inheritance of sex chromosomes (X-chromosomes; heterochromosomes), which are differentiated from the remaining autosomes. Individuals are thus allocated to sex according to the presence of one (X0) or two (XX) sex chromosomes, but although XX is usually female and X0 male, this allocation varies within and between taxonomic groups. Mechanisms involving multiple sex chromosomes also occur and there are related observations of complex fusions between sex chromosomes and autosomes. Frequently we cannot recognize sex chromosomes, particularly as sex is determined by single genes in certain insects, such as some mosquitoes and midges. Additional complications with the determination of sex arise with the interaction of both the internal and external environment on the genome (epigenetic factors). Furthermore, great variation is seen in sex ratios at birth; although the ratio is often one male to one female, there are many deviations ranging from 100% of one sex to 100% of the other.
In haplodiploidy (male haploidy) the male sex has only one set of chromosomes. This arises either through his development from an unfertilized egg (containing half of the female chromosome complement following meiosis), called arrhenotoky (section 5.10.1), or from a fertilized egg in which the paternal set of chromosomes is inactivated and eliminated, called paternal genome elimination (as in many male scale insects). Arrhenotoky is exemplified by honey bees, in which females (queens and workers) develop from fertilized eggs whereas males (drones) come from unfertilized eggs. However, sex is determined in at least some Hymenoptera by a single gene (the complimentary sex-determining locus, characterized recently in honey bees) that is heterozygous in females and hemizygous in (haploid) males. The female controls the sex of offspring through her ability to store sperm and control fertilization of eggs. Evidence points to a precise control of sperm release from storage, but very little is known about this process in most insects. The presence of an egg in the genital chamber may stimulate contractions of the spermathecal walls, leading to sperm release.