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Translocations and sex ratio distortion in the onion fly, Hylemya antiqua (Meigen), and their relevance to genetic control

  • Vosselman, L.
Publication Date
Jan 01, 1980
Wageningen University and Researchcenter Publications
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Experiments on sex determination of the onion fly, Hylemya antiqua (Meigen), are reported in the first two chapters. The main purpose was to establish if the sex ratio distortion observed in certain crosses, was based on a mechanism which could be used for genetic purposes. Two types of males, XY 1 and XXY 2 , were found and the aberrant sex ratios appeared only to occur in progenies of XXY 2 (and some XXY 2 Y 2 ) males. A numerical variation for Y 2 was frequently observed in embryos, larval ganglion cells and especially in testes, which is caused by numerical non-disjunction in somatic cells. The sex ratio distortion is attributed to such a numerical variation of Y 2 between primordial germ cells. Some progenies with a highly distorted sex ratio in male direction descended probably from XXY 2 Y 2 males. Since an effective selection of XXY 2 Y 2 males is not possible, this type of sex ratio distortion is not suited for application in a genetic control method. Other subjects treated in chapters 1 and 2 are gynandromorphism, a polymorphism with respect to the length of the X-chromosome and X-polysomy. In the progeny of one cross a chromosome morphologically identical to Y 2 but lacking its holandric inheritance was found. From its inheritance in various crosses it could be concluded that it was nothing more than a (mutated) non-functional Y 2 chromosome.The meiotic segregation of a Y 1 -linked translocation T61 is reported in the third chapter. The high percentage of alternate segregation observed in T61-heterozygous males (22 YXY 2) was in good accordance with the high fertility of this translocation (95%). Because of an early separation (or asynapsis) of the centromeric regions of the acrocentric X and Y 2, chain quadrivalents with X and Y 2in terminal position predominated in diakinesis/ prometaphase I stages. In these chain quadrivalents adjacent l is supposed to be unstable since neither X nor Y 2have a neighbouring centromere in trans-position. The very low frequency of adjacent 2 is attributed to a preferential coorientation of homologous centromeres. The suitability of T61 for genetic sexing (preferential killing of females) is discussed.Double-translocation heterozygous T14/T61 males (2 62 Y6 26XY 2) were produced by intercrossing T14-homozygous females (2 62 66 26 2XX) and T61-heterozygous males (22 Y66XY 2) These double heterozygotes showed several segregation types but for predominated. The total frequency of duplication/deficiency gametes was 60-65%. Application possibilities for these heterozygotes in genetic control of the onion fly are discussed.The meiotic segregation of five different autosomal reciprocal translocations are given in chapter 5. For translocation T14, which has very short interstitial segments, significant differences in segregation between the sexes were found. In males the ratio of alt.: adj.1:adj.2 was about 7:3:0 and in females 8:1:3. It is assumed that the very intimate meiotic pairing in males (without chiasmata) results in relatively very short "Coorientation Determining Distances" (CDD's) between the homologous centromeres. Due to these short CDD's a preferential coorientation between homologous centromeres is expected favouring alternate and adjacent l segregation. On the contrary, in females chiasmata act very probably as reference points for coorientation. Due to variable positions of the centromeres, in T14-heterozygous females less pronounced differences in CDD's between homologous and non-homologous are expected as in males. This is suggested to be the reason that in females a coorientation between non-homologous centromeres can occur resulting in a certain frequency of adjacent 2. The very low frequencies or absence of adjacent 2 in the other four translocations is attributed to their longer interstitial segments.For translocation T14 a pure breeding stock of homozygotes could be obtained using egg-hatch percentages and cytology for the recognition of the karyotypes (chapter 6). In cage experiments under laboratory conditions the competitiveness of the translocation homozygotes was tested. Starting from an initial population consisting of equal numbers of translocation homozygotes (TT) and wild type flies (++), the karyotype frequencies were determined for three successive, non-overlapping generations. The results obtained did not suggest substantial differences in fitness between TT and ++. In field cage experiments (chapter 7) a high diapause sensitivity was found for the translocation homozygotes as well as for the laboratory strain of wild type flies (++) used. As a consequence of this high diapause sensitivity the fitness of the homozygotes could not be determined under field conditions.In the "General Discussion" different application possibilities of translocations for genetic control of the onion fly are discussed. A summary of unpublished data concerning the egg-hatch and pupal survival of eight threechromosome double-translocation heterozygotes and three more complex translocation heterozygotes is also presented there. The aim of these experiments was to determine if perhaps a double translocation could be isolated meeting the conditions for a stable equilibrium. However, neither of these translocations met these conditions. Firstly, the frequencies of duplication/deficiency gametes were not high enough; secondly, insufficient complementation occurred and, thirdly, the recombination percentages in the differential segments were too high (resulting in undesirable karyotypes). Two of three double translocations tested, appeared to be viable as homozygotes in the adult stage.

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