Mating genes in Calonectria and evidence for a heterothallic ancestral state

Abstract

The genus Calonectria includes many important plant pathogens with a wide global distribution. In order to better understand the reproductive biology of these fungi, we characterised the structure of the mating type locus and flanking genes using the genome sequences for seven Calonectria species. Primers to amplify the mating type genes in other species were also developed. PCR amplification of the mating type genes and multi-gene phylogenetic analyses were used to investigate the mating strategies and evolution of mating type in a collection of 70 Calonectria species residing in 10 Calonectria species complexes. Results showed that the organisation of the MAT locus and flanking genes is conserved. In heterothallic species, a novel MAT gene, MAT1-2-12 was identified in the MAT1-2 idiomorph; the MAT1-1 idiomorph, in most cases, contained the MAT1-1-3 gene. Neither MAT1-1-3 nor MAT1-2-12 was found in homothallic Calonectria (Ca.) hongkongensis, Ca. lateralis, Ca. pseudoturangicola and Ca. turangicola. Four different homothallic MAT locus gene arrangements were observed. Ancestral state reconstruction analysis provided evidence that the homothallic state was basal in Calonectria and this evolved from a heterothallic ancestor.

Keywords: Cylindrocladium; MAT locus; fungal biology; fungal pathogens; mating type; phylogeny; sexual reproduction.

Fig. 1

Pairwise MAT loci comparison among eight Calonectria isolates representing seven species. Black horizontal lines represent genomic sequences. Colour coded arrows represent annotated genes. Red or blue boxes between genomic sequences indicates pairwise similarity based on BLASTn; red suggest that both regions are in the same orientation and blue are in opposite directions. Calonectria hongkongensis CMW 47271 represents the only homothallic individual containing both MAT1-1 and MAT1-2 idiomorph.

Fig. 2

Ancestral state reconstruction of sexual thallism of 70 Calonectria species. Homothallic species are marked with an open line, heterothallic species are marked with a solid line. Green, purple, blue and yellow coded arrows represent the MAT1-1-1, MAT1-1-3, MAT1-2-1 and MAT1-2-12 gene, respectively.

Fig. 3

Evolution models of mating type in Calonectria spp.: Heterothallic origin hypothesis. a–d. Four scenarios under which the mating type loci of heterothallic ancestors undergo an independent recombination event (unequal crossing over), resulting in the present homothallic mating type locus.

Fig. 4

Evolution models of mating type in Calonectria spp.: Homothallic origin hypothesis. a. Primary homothallic ancestor mating type locus undergoes two deletions events (gene loss) and this results in the mating type locus of two heterothallic offspring; b–d. primary homothallic ancestor mating type locus undergoes an independent deletion event which results in the present homothallic mating type locus.