Extensive divergence between mating-type chromosomes of the anther-smut fungus

Abstract

Genomic regions that determine mating compatibility are subject to distinct evolutionary forces that can lead to a cessation of meiotic recombination and the accumulation of structural changes between members of the homologous chromosome pair. The relatively recent discovery of dimorphic mating-type chromosomes in fungi can aid the understanding of sex chromosome evolution that is common to dioecious plants and animals. For the anther-smut fungus, Microbotryum lychnidis-dioicae (= M. violaceum isolated from Silene latifolia), the extent of recombination cessation on the dimorphic mating-type chromosomes has been conflictingly reported. Comparison of restriction digest optical maps for the two mating-type chromosomes shows that divergence extends over 90% of the chromosome lengths, flanked at either end by two pseudoautosomal regions. Evidence to support the expansion of recombination cessation in stages from the mating-type locus toward the pseudoautosomal regions was not found, but evidence of such expansion could be obscured by ongoing processes that affect genome structure. This study encourages the comparison of forces that may drive large-scale recombination suppression in fungi and other eukaryotes characterized by dimorphic chromosome pairs associated with sexual life cycles.

Figure 1

Alignment of restriction digest optical maps for the a₁ and a₂ mating-type chromosomes of Microbotryum lychnidis-dioicae (= M. violaceum isolated from Silene latifolia). Horizontal lines within each chromosome represent the distribution of sites cut by the restriction endonucleases NheI and AflII; separate restriction enzyme maps are shown in Figure S1. Blue shading indicates the two pseudoautosomal regions, connected by bold lines. Yellow shading indicates areas of weaker alignment, connected by lines, in the nonrecombining region. The alignments for the AflII optical map are included for the less stringent alignment parameters, as described in the text, where the default parameters produced alignments only for the pseudoautosomal regions. The a₁ and a₂ chromosomes are estimated to be ∼3.3 and 4.0 Mbp, respectively.

Figure 2

Relationship between alignment scores for regions of the NheI and AflII optical maps and alignment positions on the mating-type chromosomes of Microbotryum lychnidis-dioicae. Positions of aligned regions for the a₁ (A) and a₂ (B) chromosomes were plotted relative to the distance in kilobase pairs from the closest pseudoautosomal region; thus, the x-axis for each chromosome measures roughly half the chromosome length.