Gracilaria caudata (Gracilariales, Rhodophyta) is reproductively compatible along the whole Brazilian coast

Gracilaria spp. are economically important for the production of agar. The distribution of ecotypes of Gracilaria caudata all along the Brazilian coast raises questions regarding whether they are undergoing speciation and are possibly reproductively incompatible. Therefore, in the present work, we selected different female and male gametophytes of G. caudata from three populations along an extended Brazilian coastline (from 3° to 23° S) to perform crossing tests and observe possible reproductive barriers. In addition, we tested post-zygotic isolation, by following meiosis, the production of haploids, and the reproduction of haploids, in the progeny of these inter-population crosses. DNA sequences of the mitochondrial cytochrome c oxidase subunit 1 gene (COI) were used to determine relationships among all gametophytes employed in the crosses and to understand inheritance of mitochondria. The crosses showed interfertility between all populations tested. Neither pre- nor postzygotic isolation was seen in G. caudata from Brazil. Individuals with haplotypes that differ by 1–5 bp (0.15–0.79% divergences) in COI were reproductively compatible. In conclusion, the exchange of genetic material among populations from a wide geographic distribution allows consideration of G. caudata as a biological species.