When Do We Call Genetically Distinct Strains Different Species? - A Cautionary Case Study of the Colletotrichum gloesporioides Species Complex

Abstract

Different criteria have been used to define fungal species. Due to the increasing availability of DNA sequence information, the phylogenetic species concept (PSC) has become popular among fungal taxonomists. However, controversies remain about the criteria used to define the "phylogenetic distinctiveness" between closely related taxa and what other criteria are needed in order to derive more robust species. In a recent study, we reported the DNA sequences of four gene loci for each of 199 strains of the fungal pathogen Colletotrichum gloeosporioides species complex (CGSC) causing leaf anthracnose on the tea-oil tree Camellia oleifera in southern China. Our combined sequence analyses clustered 194 of the 199 isolates into four of the 22 previously described species within CGSC: Colletotrichum fructicola (167 isolates), Colletotrichum siamense (19 isolates), Colletotrichum gloeosporioides (6 isolates) and Colletotrichum camelliae (2 isolates). The remaining five isolates clustered distinctly from all 22 known species. Using the data, here we further investigate the extent of reproductive isolation among these species in our samples. Our analyses revealed extensive allele -sharing among three of the "phylogenetic species" and between them and the five unassigned isolates. Furthermore, there was extensive evidence for prevalent recombination among three of the four species. Our results suggest that caution should be taken in naming new species and highlight the importance of large-scale population genetic studies in helping define reproductive isolation and species boundaries.

Fusarium Species and Fusarium oxysporum Species Complex Genotypes Associated With Yam Wilt in South-Central China

“…Furthermore, both the allelic association analyses and the STRUCTURE analyses indicated allelic sharing at the six SSR loci and evidence for genetic exchanges at the population level among the species within FOSC. Thus, caution is needed in calling different genotypes as different species and more evidence should be gathered, preferably based on whole genome sequences, before further division of the FOSC into additional new species are proposed (Xu et al, 2016;Xu, 2020).…”