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The effect of transposon Pokey insertions on sequence variation in the 28S rRNA gene of Daphnia pulex

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Abstract:

The goal of this study was to determine the impact of breeding system and the presence of the transposon Pokey on intraindividual variation in 28S rRNA genes. We PCR-amplified, cloned, and sequenced 1000 nucleotides downstream of the Pokey insertion site in genes with and without insertions from 10 obligately and 10 cyclically parthenogenetic isolates of Daphnia pulex. Variation among genes with Pokey insertions was higher than variation among genes without insertions in both cyclic and obligate isolates. Although the differences were not quite significant (p = 0.06 in both cases), the results suggest that Pokey insertions are likely to inhibit the homogenization of their host genes to some extent. We also observed that the complement of 28S rRNA alleles differed between genes with and without inserts in some isolates, suggesting that a particular inserted gene can persist for substantial periods of time and even spread within the rDNA array, despite the fact that insertions are deleterious. This apparently contradictory pattern can be explained if homogenization of rRNA genes occurs primarily by gene conversion, but copies with Pokey inserts can occasionally increase in frequency within arrays owing to unequal crossing over events that do not originate in the inserted genes themselves.

Le but de ce travail était de déterminer l’impact du mode de reproduction et de la présence du transposon Pokey sur la variation intra-individuelle parmi les gènes codant pour les ARNr 28S. Les auteurs ont amplifié, cloné et séquencé 1 000 nucléotides en aval du site d’insertion de Pokey dans des gènes avec ou sans insertion chez 10 isolats parthénogéniques obligatoires et chez 10 isolats parthénogénique cycliques du Daphnia pulex. La variation au sein des gènes contenant des insertions Pokey était plus grande qu’au sein de gènes sans insertion tant chez les isolats cycliques qu’obligatoires. Bien que les différences n’étaient pas tout à fait significatives (p = 0,06 dans les deux cas), les résultats suggèrent que les insertions Pokey pourraient inhiber l’homogénéisation de leurs gènes hôtes jusqu’à un certain point. Les auteurs ont également observé que le complément d’allèles d’ARNr 28S différait entre gènes avec et sans insertion chez certains isolats, ce qui suggère qu’un gène particulier comprenant une insertion peut persister pour des périodes substantielles de temps et même se répandre au sein de l’amas d’ADNr, malgré le fait que ces insertions soient délétères. Ces observations apparemment contradictoires pourraient s’expliquer si l’homogénéisation des gènes d’ARNr se produit principalement par conversion, mais que les copies avec des insertions de Pokey peuvent occasionnellement se multiplier au sein des amas par suite d’événements de recombinaison inégale qui n’ont pas leur origine au sein des gènes dotés d’insertions.

Document Type: Research Article

Publication date: December 1, 2008

More about this publication?
  • From its inception in 1957, this international cytogenetics journal has catered to the research areas of the members of the Genetics Society of Canada; traditionally, these have included agriculture, entomology, genetics/cytogenetics, and evolutionary mechanisms. The contents of the journal have evolved as contributors developed new technologies and interests. A 20-member Editorial Board is composed of scientists from around the world. Reviews and commentary from respected experts are often featured.
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