The pseudoautosomal regions (PARs)[1] are the only homologous segments of mammalian sexual chromosomes which pair and recombine at meiosis [2]. PAR2 is human specific and lies at the tip of the long arms (Xq and Yq). It stands out from the PAR1 (located at the end of short arm of the same chromosomes) in length, gene content and especially in function. PAR2 recombines with a much lower frequency than PAR1 and is not necessary for male fertility. More striking, two out of four PAR2 genes achieve dosage compensation in a peculiar manner. While the more telomeric genes, IL9R and CXYorf1, have the biallelic expression characteristic of PAR1 genes, SYBL1/VAMP7 and SPRY3, were found to be inactivated on both the inactive X (in females) and Y chromosomes. Previous studies showed that PAR2 originated from chromosomal translocations from autosomes to the tip of Xq during Eutherian evolution and a final transposition on the end of Yq occurred only in humans [3,4]. In this study we clarify the origin and evolution of PAR2 using a comparative genomic approach. With respect to previous studies [3,4] our data set includes ten more species for which sequence data is now available, and high quality genetic data from a Neanderthal individual, representative of ancient extinct hominids [5]. In the present work we use a phylogenetic approach to reconstruct the evolution of the PAR2 region. In Marmoset (new world monkey), we discovered the central part of PAR2 is specific to Y-chromosome and we are currently validating this result by FISH analysis. This pattern is specific to Marmoset among Eutherians. We also evaluated the identity/similarity and ratio of non-synonymous to synonymous mutations for all pairs of species, observing low substitution rate between closely related species in accordance to phylogenetic analysis. Altogether, these results give new insights about the complex stepwise rearrangement occurred in PAR2 over time. The data available for the new species fill gaps in previous analyses, and this approach combined to the availability of sequence data would be useful to reconstruct the evolution of other chromosomes as well. Finally, it seems unlikely that the differences in transcriptional behaviour between the pairs of human PAR2 genes could be ascribed to their evolutionary history.
The evolutionary history of the second pseudoautosomal region revised
Marcella Vacca;Maurizio D'Esposito
2014
Abstract
The pseudoautosomal regions (PARs)[1] are the only homologous segments of mammalian sexual chromosomes which pair and recombine at meiosis [2]. PAR2 is human specific and lies at the tip of the long arms (Xq and Yq). It stands out from the PAR1 (located at the end of short arm of the same chromosomes) in length, gene content and especially in function. PAR2 recombines with a much lower frequency than PAR1 and is not necessary for male fertility. More striking, two out of four PAR2 genes achieve dosage compensation in a peculiar manner. While the more telomeric genes, IL9R and CXYorf1, have the biallelic expression characteristic of PAR1 genes, SYBL1/VAMP7 and SPRY3, were found to be inactivated on both the inactive X (in females) and Y chromosomes. Previous studies showed that PAR2 originated from chromosomal translocations from autosomes to the tip of Xq during Eutherian evolution and a final transposition on the end of Yq occurred only in humans [3,4]. In this study we clarify the origin and evolution of PAR2 using a comparative genomic approach. With respect to previous studies [3,4] our data set includes ten more species for which sequence data is now available, and high quality genetic data from a Neanderthal individual, representative of ancient extinct hominids [5]. In the present work we use a phylogenetic approach to reconstruct the evolution of the PAR2 region. In Marmoset (new world monkey), we discovered the central part of PAR2 is specific to Y-chromosome and we are currently validating this result by FISH analysis. This pattern is specific to Marmoset among Eutherians. We also evaluated the identity/similarity and ratio of non-synonymous to synonymous mutations for all pairs of species, observing low substitution rate between closely related species in accordance to phylogenetic analysis. Altogether, these results give new insights about the complex stepwise rearrangement occurred in PAR2 over time. The data available for the new species fill gaps in previous analyses, and this approach combined to the availability of sequence data would be useful to reconstruct the evolution of other chromosomes as well. Finally, it seems unlikely that the differences in transcriptional behaviour between the pairs of human PAR2 genes could be ascribed to their evolutionary history.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
