Mode of action, origin and structure of the Paternal Sex Ratio chromosome in the parasitoid wasp Trichogramma kaykai

Selfish genetic elements are defined as genetic elements that have a replication advantage relative to the rest of the genome. They are ubiquitous in nature and were extensively reported for almost all species studied so far. A special type of selfish genetic element, the sex ratio distorter, is most frequent in arthropods and changes the offspring sex ratio of its host. An example of an extremely selfish male biasing sex ratio distorter is the paternal sex ratio (PSR) chromosome in the parasitoid wasps Trichogramma kaykai and Nasonia vitripennis . These wasps have an arrhenotokous sex determination in which fertilized diploid eggs develop into females and males develop from unfertilized haploid eggs. Only part of the male wasps contains this additional B chromosome, which upon fertilizationeliminates the paternal genome, while keeping itself and the maternal chromosomes intact. The resulting haploid embryo develops into a B chromosome-carrying male. This extremely selfish B chromosome was first discovered in N. vitripennis . The recent discovery of a second PSR chromosome in the unrelated wasp T. kaykai provided an opportunity for a comparative study on PSR chromosomes. In this study I determined the mechanism, the origin and structure of the PSR chromosome in T. kaykai and compared my results with previous studies on the PSR chromosome in N. vitripennis .The mode of action of the Trichogramma PSR chromosome was revealed by examining microscopic preparations of freshly fertilized eggs. This chromosome modifies the paternal genome into a dense chromatin mass at the beginning of the first mitotic division, while the PSR chromosome itself escapes its own destructive effect and continues embryo development with the maternal chromosomes. Comparing the modes of action of the Trichogramma and Nasonia PSR chromosomes suggests that both systems are identical, except for the diameter of the paternal chromatin mass (PCM) and the occurrence of PCM-associated nuclei. However, their molecular mechanism remains unknown. Furthermore, both PSR chromosomes share the main structural characteristics of B chromosomes: They are much smaller than the normal chromosomes and contain high amounts of transposable elements and tandem repetitive DNA. B chromosomes are thought to accumulate transposable elements because they do not recombine meiotically with the normal chromosomes. Repeats have proven to be an important factor for the transmission efficiency of the Nasonia PSR chromosomes and are also thought to correlate with the B chromosome size in general. However, none of the DNA sequences found on the Trichogramma PSR chromosome were found on the Nasonia PSR chromosome or visa versa. About two thirds of the Trichogramma PSR chromosome comprises tandem arrays of 45S ribosomal DNA (rDNA), while on the Nasonia PSR chromosome three B chromosome specific repeat families are located and one repeat family that is also present on the Nasonia genome. 45S rDNA consists of three conserved genes essential for protein processing separated by three spacer sequences that are hypervariable between species but conserved within species. These spacer sequences are therefore often used for taxonomic purposes. One of the spacer sequences of the 45S rDNA on the Trichogramma PSR chromosome, i.e. ITS2, contains at least five different sequences that resemble either the ITS2 of T. kaykai or the related T. oleae . We therefore concluded that this B chromosome originated from T. oleae or a T. oleae -like species. Retrotransposon analysis revealed that the Nasonia PSR chromosome most likely originated from the Nasonia related wasp genus Trichomalopsis . Though both PSR chromosomes have a similar mode of action, the absence of any sequence homology between both chromosomes implies different PSR chromosome ancestors. This again makes it less likely that the molecular mechanism of paternal genome loss is identical. Future studies should focus on comparing the molecular mode of action and DNA sequence homology of both PSR chromosomes and revealing the incidence of more PSR chromosomes in other haplo-diploid organisms. This will not only provide more knowledge on the mechanism of early embryogenesis and in particular on the paternal chromosome processing following fertilization, but also on the origin and evolution of PSR chromosomes. Extending our knowledge on PSR chromosomes is expected most useful for the control of pest insects with haplo-diploid sex determination systems like the Argentine ant. Without females such insect populations will quickly perish.

Bibliographic Details

Main Author:	van Vugt, J.J.F.A.
Other Authors:	van Lenteren, Joop
Format:	Doctoral thesis biblioteca
Language:	English
Subjects:	chromatin, chromosomes, diploidy, parasitoids, sex determination, sex ratio, transposable elements, trichogramma, chromatine, chromosomen, diploïdie, geslachtsbepaling, geslachtsverhouding, parasitoïden, transposons,
Online Access:	https://research.wur.nl/en/publications/mode-of-action-origin-and-structure-of-the-paternal-sex-ratio-chr
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