Dmitry A. Filatov

Low variability in a Y-linked plant gene and its implications for Y-chromosome evolution.

Sex chromosomes have evolved independently in several different groups of organisms, but they share common features, including genetic degeneration of the Y chromosome. Suppression of recombination between ancestral proto-X and proto-Y chromosomes is thought to have led to their gradual divergence, and to degeneration of the Y chromosome, but the evolutionary forces responsible are unknown. In non-recombining Y chromosomes, deleterious mutations may be carried to fixation by linked advantageous mutations (“selective sweeps”). Occurrence of deleterious mutations may drive “Mullers ratchet” (stochastic loss of chromosomes with the fewest mutations). Selective elimination of deleterious mutations, causing “background selection” may accelerate stochastic fixation of mildly detrimental mutations. All these processes lower effective population sizes, and therefore reduce variability of genes in evolving Y chromosomes. We have studied DNA diversity and divergence in a recently described X- and Y-linked gene pair (SLX-1 and SLY-1) of the plant Silene latifolia to obtain evidence about the early stages of Y degeneration. Here we show that DNA polymorphism in SLY-1 is 20-fold lower than in SLX-1, but the pattern of polymorphism does not suggest a selective sweep.

THE RELATIONSHIP BETWEEN THE RATE OF TRANSPOSITION AND TRANSPOSABLE ELEMENT COPY NUMBER FOR COPIA AND DOC RETROTRANSPOSONS OF DROSOPHILA MELANOGASTER

We present data on the relationship between the rate of transposition and copy number in the genome for the copia and Doc retrotransposons of Drosophila melanogaster. copia and Doc transposition rates were directly measured in sublines of the isogenic 2b line using individual males or females, respectively, with a range of copia copy numbers from 49 to 103 and Doc copy numbers from 112 to 235 per genome. Transposition rates varied from 3 x 10(-4) to 2 x 10(-2) for copia and from 2 x 10(-4) to 2 x 10(-3) for Doc. A positive relationship between transposition rate and copy number was found both for copia and for Doc when the data were analysed across all the 2b individuals; no significant correlation was found when the data were analysed across the subline means for both retrotransposons tested. Overall, correlation between copia and Doc transposition rate and their copy number in the genome, if any, was not negative, which would be expected if transposable elements (TEs) self-regulate their copy number. Thus, for copia and Doc no evidence for self-regulation was provided, and at least for these two TEs this hypothesis is not favoured for explaining the maintenance of the stable copy number that is characteristic for natural populations. The transposition rate of copia was measured twice, and a strong positive correlation between copy number and transposition rate both across individuals and subline means was found in 1994, while in 1995 no correlation was found. This fact is in agreement with the hypothesis that a positive correlation between the rate of transposition and TE copy number may be a default starting point for future host-TE coevolution.

AGE DEPENDENCE OF THE COPIA TRANSPOSITION RATE IS POSITIVELY ASSOCIATED WITH COPIA TRANSCRIPT ABUNDANCE IN A DROSOPHILA MELANOGASTER ISOGENIC LINE

Abstract In males of an inbred Drosophila melanogaster line (2b), a significant age dependence of the transposition rate of the retrotransposon copia was noted. Among males of seven different age groups, the lowest transposition rate was detected in 12- to 15-day-old males, and the highest transposition rates were observed in 1-␣to 3- and in 24- to 27-day-old males: rates of 0.36 ± 0.07, 1.09 ± 0.20 and 117 ± 0.27 transpositions per generation per gamete, respectively, were measured. This type of age dependence of the transposition rate indicates that copia transpositions do not occur in stem cells. Clusters of copia transpositions were detected, which confirms that copia transpositions occur at premeiotic stages of spermatogenesis. A positive association between copia transposition rate and copia transcript abundance in the testes was demonstrated for 1- to 3-, 12- to 15- and 24- to 27-day-old males, as well as for 1- to 3-day-old males of two closely related sublines of the 2b line that differed in copia transposition rate. Thus, copia transposition rate in males is apparently regulated at the level of copia RNA abundance in testes. The age dependence of the copia transposition rate and copia transcript abundance within the 2b line demonstrates that the physiological state of the host may affect expression and transposition of the retroelement.

Impact of the Regulatory Regions of Retrotransposon copia on the Level of Its Expression in Testes of Drosophila melanogaster

Expression of the lacZ reporter gene under the control of five deletion derivatives of the copia regulatory region including the 5′ long terminal repeat (LTR) and the 5′ untranslated region (UTR) was assayed in the testes of transgenic Drosophila melanogaster males (larvae and imago). The full-length copia regulatory region (LTR + UTR) ensured expression of the reporter gene in testes of both larvae and adult males. Deletion of UTR or 3′ end of LTR increased lacZ expression in the testes, whereas deletion of the 5′ end of LTR increased it. This indicated that a positive regulator ofcopia expression is at the 5′ end of LTR and that negative regulators are at the 3′ end of LTR and in UTR. The effects of the fragments of the copia regulatory region on reporter gene expression in the testes in vivo did not completely coincide with the effects observed earlier in cultured cells. We suggest that this difference is due to different regulation of expression of the fusion constructs integrated into chromatin as compared to their transient expression.

Age dependence of the level of copia retrotransposon expression in the testes of Drosophila melanogaster

Expression of the lacZ reporter gene controlled by various deletion derivatives of the regulatory region of the copia retrotransposon was studied in the testes of adult transgenic males of the Drosophila melanogastery1w67c23(2) strain at the age of 3, 6–9, 12–15, 18–21, and 24–27 days. When the construct contained the full-length regulatory region, which included the 5′-long terminal repeat (LTR) and the 5′-untranslated region (UTR), expression was the lowest in males aged 12–15 days and the highest in males aged 3 or 24–27 days. A similar V-shaped age dependence was previously observed for the copia transposition rate and RNA content in a strain with a high rate of copia transposition. Thus, the V-shaped age dependence of expression, which is unusual for Drosophila, proved to be characteristic of copiaregardless of its transposition rate. Deletion of the 5′ or 3′ end of the LTR, but not of the UTR, changed the age dependence of the level of reporter gene expression. In this case, expression was the highest in 3-day-old males and gradually decreased with age, as typical for many Drosophila genes. It was assumed that the 5′- and 3′-terminal regions of the copiaLTR contain regulatory elements responsible for the V-shaped age dependence of expression, while the expression level depends to a greater extent on the regulatory elements of UTR.