Genetic diversity of sympatric Schizymenia dubyi and S. apoda (Schizymeniaceae, Rhodophyta) in Wellington Harbour, New Zealand

Schizymenia dubyi is reported as a new introduction in Wellington, which already has introduced S. apoda. These species coexist in tidepools and the crustose sporophyte is for the first time reported from this population. The diversity and species status of these two species was explored using mitochondrial, plastid and nuclear markers. Haplotypes of rbcL in New Zealand samples of S. dubyi match a haplotype found in Australia that was considered as ‘introgressed’ into S. apoda. COI haplotypes of S. dubyi from New Zealand are also similar to specimens from Australia and Europe, although New Zealand S. apoda samples have a unique haplotype. ITS ribotypes of S. dubyi from New Zealand also are shared with Australia and Europe, while again S. apoda ribotypes are unique. Phylogenetic and network analyses show that these two species are very closely related, and the hypothetical ‘introgressed’ nature of specimens is not supported by phylogenetic analyses. In addition, species delimitation methods are not congruent and do not clarify the species status of these two entities. Morphologically, these species are not distinct, except that in S. dubyi the cystocarp ostiole may be in a depression. The apparent incongruence between markers, leading to the hypothesis of introgression, is probably due to the early stages of the speciation process during which species have a ‘paraphyletic’ phase, in which stochastic incomplete lineage sorting leads to ancestral haplotypes being retained differentially, versus hybridization, introgression and non-maternal inheritance of organelles. Whether these two entities are distinct species needs to be reconsidered, as many species criteria are not met (e.g. monophyly) or need to be tested (e.g. reproductive barriers).