Research
Sex determination in insects
Sex determination cascade
Sexual determination is one of the most important developmental decisions, but comes down to one binary choice: becoming male or female. This decisions has huge consequences for the life history of the individual, especially in term of morphological, physiological and behavioural traits. Although the occurrence of only two sexes is universal in plants and animals, the molecular mechanisms for sex determination are extraordinary variable. One of the best studied groups are insects where male heterogamety (XX-XY), female heterogamety (ZW-ZZ), haplodiploidy and more remarkable mechanisms are present. The diversity of sexual phenotypes and the well studied life history traits in insects, makes them the ideal group to study variation in sex determination, the factors that shaped its evolution, and the relationship between life history and ecology of the species involved.
In insects, sex determination involves multiple steps, starting with a primary genetic signal that is different in males and females. This primary genetic signal is usually on one of the sex chromosomes and is transmitted through a cascade of genes to a master switch gene. This master switch gene then regulates all genes that are required for sex specific traits. The sex determination cascade in insects has a shape that resembles an hourglass. On top are all the different primary signal that sets the decision for the sexual development, in the centre is the axis of two important genes in sex determination: transformer and the master switch gene doublesex. At the bottom of the cascade are all the different sexual traits that are switched on during development.
In insects, sex determination involves multiple steps, starting with a primary genetic signal that is different in males and females. This primary genetic signal is usually on one of the sex chromosomes and is transmitted through a cascade of genes to a master switch gene. This master switch gene then regulates all genes that are required for sex specific traits. The sex determination cascade in insects has a shape that resembles an hourglass. On top are all the different primary signal that sets the decision for the sexual development, in the centre is the axis of two important genes in sex determination: transformer and the master switch gene doublesex. At the bottom of the cascade are all the different sexual traits that are switched on during development.
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Nasonia and Muscidifurax
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Nasonia and Muscidifurax are both 2-3 mm long parasitic wasps that lay their eggs in the pupae of a fly. Both genera belong to the sub-family Pteromalinae (Figure 1) and, as can be seen in the pictures below, the two genera are morphologically very similar. All parasitic wasps are in the order of the Hymenoptera comprising also the ants, bees and sawflies. One specific characteristic of the Hymenoptera is haplodiploidy. This means that the females are diploid and thus have two chromosome sets (just as humans), and males are haploid with only one chromosome set (see Figure 2). A virgin female (unmated) can produce only unfertilized haploid eggs that will develop into males, but if she has mated with one or more males, she can produce still haploid eggs (males) but also diploid fertilized eggs that will develop into females.
Nasonia is widely used as a model-organism because it is easy to rear and use in the lab. In 2010 the genome sequences of three Nasonia species were published in Science. In addition many molecular and genomic tools have become available in the past years, such as RNA interference, expression array data, SNP chips and QTL analyses. In the paragraph below more in depth information can be found on Nasonia vitripennis and a lot of information on the Nasonia genus is also available on the WerrenLab website. Less research has been done involving the Muscidifurax genus. The species M. raptorellus has primarily been used in biological pest control. More information on the Muscidifurax genus can be found in its subsection soon.
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Figure 1. Part of Chalcidoidea phylogeny (Image from: Munro JB, Heraty JM, Burks RA, Hawks D, Mottern J, et al. (2011) A Molecular Phylogeny of the Chalcidoidea (Hymenoptera). PLoS ONE 6(11): e27023.a)
Figure 2. Haplodiploid reproduction. The 'n' stands for one chromosome set. The black 'n' comes from the female and the white 'n' comes from the father.
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Nasonia vitripennis
Life cycle
Nasonia vitripennis is the only cosmopolitan species of the genus and has been found across the whole of the northern Palearctic region, and the US. In the US it occurs together (called sympatry) with the three other Nasonia species. N. vitripennis has no particular host preference and will parasitize a wide range of fly pupae, but mainly blowflies (Calliphoridae) and fleshflies. Some of these flies (Protocalliphora) can be found in bird nests where they drink the blood of young bird chicks. A Nasonia female locates these fly pupae and drills a hole with her ovipositor through its chitinous outer puparium. She then injects inject venom into the host, which is a complex mix of chemicals that will arrest the host development and make it suitable food for her offspring. Twenty to 40 small eggs are laid on the hosts outer integument and within 2 days, these eggs will hatch. The tiny larvae will eat from the host fly for the next 7 days, after which the larvae stop feeding and transform into pupae that will develop for another 7 days, leading to a total developmental time of from egg to adult of approximately 14 days at 25°C. The Nasonia males, which emerge several hours before the females, escape by chewing small holes in the hosts puparium.
Life cycle
Nasonia vitripennis is the only cosmopolitan species of the genus and has been found across the whole of the northern Palearctic region, and the US. In the US it occurs together (called sympatry) with the three other Nasonia species. N. vitripennis has no particular host preference and will parasitize a wide range of fly pupae, but mainly blowflies (Calliphoridae) and fleshflies. Some of these flies (Protocalliphora) can be found in bird nests where they drink the blood of young bird chicks. A Nasonia female locates these fly pupae and drills a hole with her ovipositor through its chitinous outer puparium. She then injects inject venom into the host, which is a complex mix of chemicals that will arrest the host development and make it suitable food for her offspring. Twenty to 40 small eggs are laid on the hosts outer integument and within 2 days, these eggs will hatch. The tiny larvae will eat from the host fly for the next 7 days, after which the larvae stop feeding and transform into pupae that will develop for another 7 days, leading to a total developmental time of from egg to adult of approximately 14 days at 25°C. The Nasonia males, which emerge several hours before the females, escape by chewing small holes in the hosts puparium.
Sex specific morphological differences
Adult N. vitripennis males and females are very easy to differentiate, after a little practise even without a binocular. Here, I will not go into too much detail on the differences between males, females and the species. If you would like to read all the details on the morphology of the Nasonia genus, I can recommend the paper by Darling and Werren, 1990.
The easiest observation is the size of the forewing. In females this is a full-size wing, but in males the forewings are small and they cannot fly. Note though, only in N. vitripennis is this difference very pronounced. In the other Nasonia species the male and female wings are roughly the same and both males and females can fly. The antennae are yellow in males and black in females as are the femurs. On the ventral side, a line can be seen on the abdomen of the females, this is where the ovipositor is stored. Males have no line and a much more rounded shape of the abdomen, where the females have a pointy abdomen. The male genitalia (aedeagus) is normally not visible. Males have a bright green / bronze iridescent coloration of their thorax and head that fainter in females.
The easy to tell differences are:
Adult N. vitripennis males and females are very easy to differentiate, after a little practise even without a binocular. Here, I will not go into too much detail on the differences between males, females and the species. If you would like to read all the details on the morphology of the Nasonia genus, I can recommend the paper by Darling and Werren, 1990.
The easiest observation is the size of the forewing. In females this is a full-size wing, but in males the forewings are small and they cannot fly. Note though, only in N. vitripennis is this difference very pronounced. In the other Nasonia species the male and female wings are roughly the same and both males and females can fly. The antennae are yellow in males and black in females as are the femurs. On the ventral side, a line can be seen on the abdomen of the females, this is where the ovipositor is stored. Males have no line and a much more rounded shape of the abdomen, where the females have a pointy abdomen. The male genitalia (aedeagus) is normally not visible. Males have a bright green / bronze iridescent coloration of their thorax and head that fainter in females.
The easy to tell differences are:
- Antennae color
- Leg (femur) color
- Wing size (only in N. vitripennis)
- Genitalia
- Body color
The below sections will follow...
Development
Gynandromorphs
Eye color mutants
RNA interference
Development
Gynandromorphs
Eye color mutants
RNA interference
Muscidifurax raptorellus
To be updated...
To be updated...
Muscidifurax uniraptor
To be updated...
To be updated...
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