Abstract:
The reproductive genes of fungi, like those of many other organisms, are thought to diversify
rapidly. This phenomenon could be associated with the formation of reproductive barriers and
speciation. Ascomycetes produce two classes of mating-type-specific peptide pheromones. These
are required for recognition between the mating types of heterothallic species. Little is known
regarding the diversity or the extent of species-specificity in pheromone peptides among these
fungi. We compared the putative protein-coding DNA sequences of the two pheromone classes
from 70 species of Ascomycetes. The dataset included previously-described pheromones and
putative pheromones identified from genomic sequences. In addition, pheromone genes from
twelve Fusarium species in the Gibberella fujikuroi complex were amplified and sequenced.
Pheromones were largely conserved among species in this complex and, therefore, cannot alone
account for the reproductive barriers observed between these species. In contrast, pheromone
peptides were highly diverse among many other Ascomycetes, with evidence for both positive
diversifying selection and relaxed selective constraint. Repeats of the α-factor-like pheromone,
which occur in tandem arrays of variable copy number, were found to be conserved through
purifying selection and not concerted evolution. This implies that sequence-specificity may be
important for pheromone reception and that inter-specific differences may indeed be associated
with functional divergence. Our findings also suggest that frequent duplication and loss causes
the tandem-repeats to experience “birth-and-death” evolution, which could in fact facilitate interspecific
divergence of pheromone peptide sequences.
