A critical deliberation of the 'species complex' status of the globally spread colonial ascidian Botryllus schlosseri

The accurate taxonomic identity for the worldwide-distributed invasive ascidian Botryllus schlosseri has not been resolved. Employing molecular toots, primarily mtDNA, previous studies unveiled five divergent clades (A-E), suggesting a complex of five cryptic species. A recent study allocated clades A and E to different species. Here, worldwide B. schlosseri's COI distribution map has been drawn, based on 2927 specimens, elucidating 160 haplotypes (100 singletons). Glade A emerged as the most abundant and globally widespread, while other clades had more limited distributions (primarily B, C). Inter-Glade and intra-clade divergences were similar, with no clear barcoding gaps between the clades, illuminating no more than two putative OTUs. Network analyses for the genetic similarities among the clades' haplotypes identified different groups, depending on threshold values and away from the suggested clades' boundaries. Three additional genetic markers (H3, 18S, 28S) disclosed Glade A, segregating from other clades and clades D and E strongly integrating. Allorecognition assays between clades resulted in indifference and rejection outcomes, characteristics of the within-species allorecognition repertoire. The question as to whether Botryllus schlosseri is a single species or a species complex is further discussed, leading to the assertion that while it is a widely variable species, there is not enough evidence for its designation as a species complex.

Automated summary

This study aimed to determine the taxonomic identity of the invasive ascidian Botryllus schlosseri, which is widely distributed worldwide. The results showed the presence of five different clades, with clade A being the most abundant and widely distributed. Other clades had more limited distributions. The genetic differences between and within clades were similar, and there were no clear barcoding gaps between the clades, indicating the presence of no more than two putative operational taxonomic units (OTUs). Additional genetic markers supported the integration of clade A with other clades and clades D and E. Allorecognition assays revealed indifference and rejection outcomes, indicating within-species allorecognition. The discussion suggests that while Botryllus schlosseri is a highly variable species, there is not enough evidence to designate it as a species complex.