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Evidence of the accumulation of allele-specific non-synonymous substitutions in the young region of recombination suppression within the mating-type chromosomes of Neurospora tetrasperma.

ABSTRACT: Currently, little is known about the origin and early evolution of sex chromosomes. This is largely due to the fact that ancient non-recombining sex chromosomes are highly degenerated, and thus provide little information about the early genomic events in their evolution. The Neurospora tetrasperma mating-type (mat) chromosomes contain a young (<6 Mya) and large region (>6.6?Mb) of suppressed recombination, thereby providing a model system to study early stages of sex chromosome evolution. Here, we examined alleles of 207 genes located on the N. tetrasperma mat a and mat A chromosomes to test for signs of genomic alterations at the protein level in the young region of recombination suppression. We report that the N. tetrasperma mat a and mat A chromosomes have each independently accumulated allele-specific non-synonymous codon substitutions in a time-dependent, and gene-specific manner in the recombinationally suppressed region. In addition, examination of the ratio (?) of non-synonymous substitutions (dN) to synonymous substitutions (dS) using maximum likelihood analyses, indicates that such changes are associated with relaxed purifying selection, a finding consistent with genomic degeneration. We also reveal that sex specific biases in mutation rates or selection pressures are not necessary for genomic alterations in sex chromosomes, and that recombination suppression in itself is sufficient to explain these results. The present findings extend our current understanding of genomic events associated within the young region of recombination suppression in these fungal sex-regulating chromosomes.

SUBMITTER: Whittle CA

PROVIDER: S-EPMC3182494 | biostudies-literature | 2011 Oct

REPOSITORIES: biostudies-literature

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Evidence of the accumulation of allele-specific non-synonymous substitutions in the young region of recombination suppression within the mating-type chromosomes of Neurospora tetrasperma.

Whittle C A CA Johannesson H H

Heredity 20110309 4

Currently, little is known about the origin and early evolution of sex chromosomes. This is largely due to the fact that ancient non-recombining sex chromosomes are highly degenerated, and thus provide little information about the early genomic events in their evolution. The Neurospora tetrasperma mating-type (mat) chromosomes contain a young (<6 Mya) and large region (>6.6 Mb) of suppressed recombination, thereby providing a model system to study early stages of sex chromosome evolution. Here, ...[more]

PMID: 21386869

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Project description:The significance of introgression as an evolutionary force shaping natural populations is well established, especially in animal and plant systems. However, the abundance and size of introgression tracts, and to what degree interspecific gene flow is the result of adaptive processes, are largely unknown. In this study, we present medium coverage genomic data from species of the filamentous ascomycete Neurospora, and we use comparative genomics to investigate the introgression landscape at the genomic level in this model genus. We revealed one large introgression tract in each of the three investigated phylogenetic lineages of Neurospora tetrasperma (sizes of 5.6 Mbp, 5.2 Mbp, and 4.1 Mbp, respectively). The tract is located on the chromosome containing the locus conferring sexual identity, the mating-type (mat) chromosome. The region of introgression is confined to the region of suppressed recombination and is found on one of the two mat chromosomes (mat a). We used Bayesian concordance analyses to exclude incomplete lineage sorting as the cause for the observed pattern, and multilocus genealogies from additional species of Neurospora show that the introgression likely originates from two closely related, freely recombining, heterothallic species (N. hispaniola and N. crassa/N. perkinsii). Finally, we investigated patterns of molecular evolution of the mat chromosome in Neurospora, and we show that introgression is correlated with reduced level of molecular degeneration, consistent with a shorter time of recombination suppression. The chromosome specific (mat) and allele specific (mat a) introgression reported herein comprise the largest introgression tracts reported to date from natural populations. Furthermore, our data contradicts theoretical predictions that introgression should be less likely on sex-determining chromosomes. Taken together, the data presented herein advance our general understanding of introgression as a force shaping eukaryotic genomes.

Dataset Information

Evidence of the accumulation of allele-specific non-synonymous substitutions in the young region of recombination suppression within the mating-type chromosomes of Neurospora tetrasperma.

Publications

Evidence of the accumulation of allele-specific non-synonymous substitutions in the young region of recombination suppression within the mating-type chromosomes of Neurospora tetrasperma.

Similar Datasets

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