Fine Characterization of the Contact Zones of <i>Mazzaella laminarioides</i> in the Southeastern Pacific Using Mitochondrial and Nuclear Markers

In <i>Mazzaella laminarioides</i>, a red alga commonly encountered along Chilean rocky coasts, three genetic lineages, named North, Center, and South and presenting distinct levels of genetic divergence, have been previously described based on organellar sequences. However, the coarse sampling strategy and the type of molecular markers used limited the understanding of where, when and how the lineages interact. Here we present the results obtained for six nuclear microsatellites and the mitochondrial COI acquired for 29 localities positioned between the 28°S and the 43°S, allowing us for the first time to study precisely the position of the contact zones between the three lineages. We also tested for signals potentially related to introgression between lineages using phylogenetic network reconstruction and sequences of eight nuclear loci. Our results show that the three genetic lineages of <i>M. laminarioides</i> already represent highly genetically isolated sibling species. Indeed, the level of admixture estimated with STRUCTURE detected only a few introgressed individuals even when lineages are located a few kilometers apart. The transition zone detected between North and Center corresponds to a more or less continuous rocky platform of only 5 km in length located at 32°48'S. Contrastingly, a mosaic of Center and South sampling sites is observed in an area extending over some 225 km between 37°S and 38°S; generating three transition zones between these two lineages, all centered on sandy beaches. We propose that a complex history of earthquakes, coastal uplift, and subsequent recolonization events is at the origin of this Center and South mosaic. We propose that these results reflect the contrasting levels of reproductive isolation that have been established between the lineages. The split between North and Center/South lineages being much older than the Center South one, intrinsic barriers may have already developed between North and Center while the limitation of gene flow between Center and South still mostly relies on extrinsic barriers.