Fertility depression among cheese-making Penicillium roqueforti strains suggests degeneration during domestication

Abstract

Genetic differentiation occurs when gene flow is prevented, due to reproductive barriers or asexuality. Investigating the early barriers to gene flow is important for understanding the process of speciation. Here, we therefore investigated reproductive isolation between different genetic clusters of the fungus Penicillium roqueforti, used for maturing blue cheeses, and also occurring as food spoiler or in silage. We investigated premating and postmating fertility between and within three genetic clusters (two from cheese and one from other substrates), and we observed sexual structures under scanning electron microscopy. All intercluster types of crosses showed some fertility, suggesting that no intersterility has evolved between domesticated and wild populations despite adaptation to different environments and lack of gene flow. However, much lower fertility was found in crosses within the cheese clusters than within the noncheese cluster, suggesting reduced fertility of cheese strains, which may constitute a barrier to gene flow. Such degeneration may be due to bottlenecks during domestication and/or to the exclusive clonal replication of the strains in industry. This study shows that degeneration has occurred rapidly and independently in two lineages of a domesticated species. Altogether, these results inform on the processes and tempo of degeneration and speciation.

Keywords: Purifying selection; postzygotic; prezygotic; reproductive isolation; sex evolution; speciation; species criteria; sterility.

Figure 1

Crosses between strains of Pencillium roqueforti of opposite mating types. (A) inoculation scheme for setting up crosses between two strains. (B) Pictures of more or less fertile crosses. Cleistothecia were absent from infertile crosses (B1) while successful crosses (B2, B3) showed cleistothecia along junctions of intersecting colonies (white arrows). The strains used in the B2 and B3 crosses display contrasted mycelium morphologies but these two crosses were fully fertile, with no obvious difference in their cleistothecia or ascospore morphologies. The strain IDs are indicated in colors corresponding to those of the clusters in Figures 2 and S2: yellow indicates W–C– cheese strains, blue W+C+ cheese strains, and red noncheese strains.

Figure 2

Histograms showing the genetic diversity (mean of the expected heterozygosity H_E, mean number of alleles per marker, total number of alleles across all markers) and linkage disequilibrium levels in the three genetic clusters of Penicillium roqueforti, corresponding to W–C– cheese strains, W+C+ cheese strains and noncheese strains, respectively. Colors correspond to those in Figures 1 and S2.

Figure 3

Results of the different types of crosses (within and between the three genetic clusters of Penicillium roqueforti, including W–C– cheese strains, W+C+ cheese strains and noncheese strains, respectively). In black is shown the proportion of crosses with no cleistothecia (premating intersterility), in gray the proportion of crosses with cleistothecia but without ascospores (postmating intersterility), and in white the proportion of crosses with ascospores in the cleistothecia (full fertility).

Figure 4

Sexual and asexual structures of Penicillium roqueforti taken using scanning electron microscopy. (A) Conidiophore, bearing asexual spores (conidia) on the plate of the cross 6248 × 3969 (W+C+ cheese × noncheese); the white bar corresponds to 2 μm. (B) Cleistothecia, from the cross 6248 × 3969 (W+C+ cheese × noncheese); the white bar corresponds to 200 μm. (C) Ascospores and asci from the cross 6201 × 6145 (W–C– cheese × W–C– cheese); the white bar corresponds to 10 μm; asci are indicated by the letter “a” and ascospores by white arrows. (D) Ascospores in an ascus in the cross 6133 × 6145 (W+C+ cheese × W–C– cheese); the white bar corresponds to 2 μm. (E) Ascospores in the cross 6201 × 6145 (W–C– cheese × W–C– cheese); the white bar corresponds to 2 μm. (F) Ascospores in the cross 6037 × 6136 (noncheese × W–C– cheese); the white bar corresponds to 2 μm.