Academic journal article Genetics

No Patrigenes Required for Femaleness in the Haplodiploid Wasp Nasonia Vitripennis

Academic journal article Genetics

No Patrigenes Required for Femaleness in the Haplodiploid Wasp Nasonia Vitripennis

Article excerpt

ABSTRACT

The parasitoid wasp Nasonia vitripennis is an emerging model organism for developmental and behavioral genetics. It reproduces by haplodiploidy; males typically develop parthenogenetically from haploid eggs and females from fertilized diploid eggs. A polyploid mutant strain is available in which females are triploid and lay haploid and diploid eggs that normally develop into males when unfertilized. In contrast to previous reports, ~2% of triploid females were found to occasionally produce daughters as well as gynandromorphs from diploid unfertilized eggs. Daughter production increased with age and differed among familial lineages. This is the first report of parthenogenetic female development in Nasonia. The results show that a paternally provided genome is not required for femaleness and call for modifications of existing models of sex determination in Nasonia.

ALL Hymenoptera (ants, bees, and wasps) have haplodiploid sex determination. The most common mode of reproduction is arrhenotoky; i.e., males develop from unfertilized eggs and are haploid, whereas females develop from fertilized eggs and are diploid. As a consequence, males inherit genes from their mother only and have no genetic father. Thelytoky also is widespread among Hymenoptera. This reproductive mode refers to species that produce females parthenogenetically in the absence of males. Parthenogenesis is automictic; i.e., haploid eggs are produced meiotically but undergo diploidy restoration by a variety of mechanisms (SuOMALAINEN et al. 1987). Hence, these diploid eggs develop into uniparental females.

In haplodiploids there are no heteromorphic sex chromosomes; the only genetic difference between males and females is the number of chromosome sets. The question of how this difference in copy number of the genome can lead to the development of either males or females can still not be answered satisfactorily. A number of models have been proposed for sex determination in Hymenoptera and available data indicate that at least two different mechanisms exist (reviewed in COOK 1993a; DOBSON and TANOUYE 1998).

WHITING (1943) introduced angle focus complementary sex determination (sl-CSD) based on his studies of the parasitoid Bracon hebetor. Sex is determined by a single locus; hétérozygotes develop into females and hemizygotes and homozygotes develop into males. This mode of sex determination is typically detected with inbreeding crosses, which lead to diploid homozygous males (WHITING 1943; STOUTHAMER et al. 1992; COOK 1993a,b). A sl-CSD mechanism of sex determination has now been shown for >40 species of Hymenoptera (reviewed in STOUTHAMER et al. 1992; COOK 1993a; PERIQUET et al. 1993; BUTCHER et al. 2000), covering all major suborders. However, the precise phylogenetic distribution of this mode of sex determination is still unclear (BEUKEBOOM and PIJNACKER 2000). Moreover, although the sex locus has recently been characterized from the honey bee (BEYE et al. 2003), the precise molecular genetic mechanism of sl-CSD remains to be elucidated.

In a number of hymenopteran species inbreeding does not result in diploid males and indicates the absence of a sl-CSD mechanism (SKINNER and WERREN 1980; COOK 1993b; BEUKEBOOM and PIJNACKER 2000). sl-CSD is also incompatible with most forms of thelytoky. Peri-meiotic mechanisms that include union of meiotic products, such as central and terminal fusion, cause an increase of homozygosity. Homozygosity of the csrfgene leads to diploid males rather than females. Therefore, CSD mechanisms can be reconciled with thelytoky only if the csd gene resides in a region that remains permanently heterozygous (BEUKEBOOM and PIJNACKER 2000). Wolbachia-induced thelytoky (STOUTHAMER et al. 1990; BRAIG et al. 2002) causes gamete duplication leading to complete homozygosity and therefore cannot occur in species with CSD.

The parasitoid wasp Nasonia vitripennis is one of the best-studied hymenopterans genetically. …

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