Gene genealogies indicates abundant gene conversions and independent evolutionary histories of the mating-type chromosomes in the evolutionary history of Neurospora tetrasperma

Abstract

Background: The self-fertile filamentous ascomycete Neurospora tetrasperma contains a large (approximately 7 Mbp) and young (< 6 MYA) region of suppressed recombination within its mating-type (mat) chromosomes. The objective of the present study is to reveal the evolutionary history, including key genomic events, associated with the various regions of the mat chromosomes among ten strains representing all the nine known species (lineages) contained within the N. tetrasperma species complex.

Results: Comparative analysis of sequence divergence among alleles of 24 mat-linked genes (mat A and mat a) indicates that a large region of suppressed recombination exists within the mat chromosome for each of nine lineages of N. tetrasperma sensu latu. The recombinationally suppressed region varies in size and gene composition among lineages, and is flanked on both ends by normally recombining regions. Genealogical analyses among lineages reveals that eight gene conversion events have occurred between homologous mat A and mat a-linked alleles of genes located within the region of restricted recombination during the evolutionary history of N. tetrasperma.

Conclusions: We conclude that the region of suppressed recombination in the mat chromosomes has likely been subjected to independent contraction and/or expansion during the evolutionary history of the N. tetrasperma species complex. Furthermore, we infer that gene conversion events are likely a common phenomenon within this recombinationally suppressed genomic region. We argue that gene conversions might provide an efficient mechanism of adaptive editing of functional genes, including the removal of deleterious mutations, within the young recombinationally suppressed region of the mat chromosomes.

Figure 1

Schematic illustration of the mating-type (mat) chromosomes of nine Neurospora tetrasperma lineages. Approximate location of investigated genes, the mat locus and the centromere are indicated. Horizontal bars show the region of lineage-specific expansion of non-recombining region and instances of gene conversions and crossovers between the two chromosomes in nine lineages of N. tetrasperma.

Figure 2

Gene genealogies of 16 genes in the region of suppressed recombination of the mating-type (mat) chromosomes of Neurospora tetrasperma. The genes are visualized horizontally in a consecutive order according to gene order in Neurospora crassa, from left to right flank of the chromosomes. The position of the mat locus and the centromere are indicated. Alleles from mat chromosomes originating from wild-type heterokaryons are indicated by strain ID given in Table 1, and strains of the same phylogenetic species in the N. tetrasperma species complex are marked with the same color. Gene conversion events are boxed and indicated by yellow stars. Alleles of genes marked in bold italic were used as template for gene conversion.