Cryptic diversity in southern African kelp

Abstract

The southern coast of Africa is one of the few places in the world where water temperatures are predicted to cool in the future. This endemism-rich coastline is home to two sister species of kelps of the genus Ecklonia maxima and Ecklonia radiata, each associated with specific thermal niches, and occuring primarily on opposite sides of the southern tip of Africa. Historical distribution records indicate that E. maxima has recently shifted its distribution ~ 70 km eastward, to sites where only E. radiata was previously reported. The contact of sister species with contrasting thermal affinities and the occurrence of mixed morphologies raised the hypothesis that hybridization might be occurring in this contact zone. Here we describe the genetic structure of the genus Ecklonia along the southern coast of Africa and investigate potential hybridization and cryptic diversity using a combination of nuclear microsatellites and mitochondrial markers. We found that both species have geographically discrete genetic clusters, consistent with expected phylogeographic breaks along this coastline. In addition, depth-isolated populations were found to harbor unique genetic diversity, including a third Ecklonia lineage. Mito-nuclear discordance and high genetic divergence in the contact zones suggest multiple hybridization events between Ecklonia species. Discordance between morphological and molecular identification suggests the potential influence of abiotic factors leading to convergent phenotypes in the contact zones. Our results highlight an example of cryptic diversity and hybridization driven by contact between two closely related keystone species with contrasting thermal affinities.

Figure 1

Sampling locations of Ecklonia in southern Africa and genetic structure inferred from mtDNA and microsatellite genotypes. Warm colors (orange–yellow) identify E. radiata haplogroups and clusters while cold colors (blue–purple) identify E. maxima. Colored bars above STRUCTURE plots indicate individuals’ haplotype. Black diamond marks denote individuals for which there was a discordant species attribution between mitochondrial and nuclear markers. A) Geographic distribution of COX1 haplotypes (pie charts) and STRUCTURE clusters (borders) for K = 8 and B) haplotype network showing haplotype genealogy. C) Structure plots assuming K = 2 (top) and K = 4 (middle) and species specific (bottom) genetic clusters and D) FCA scatter plot based on the clusters defined K = 4, circles colored according to mtDNA haplotypes present in the cluster. Map was created using a custom R (https://cran.r-project.org/) script on version 4.3.0 and edited using Affinity Designer, version 2.3.1.

Figure 2

Potential distribution of Ecklonia radiata and Ecklonia maxima predicted with Species Distribution Modelling. Predictions aggregated in equal area hexagons for better visualization. The model correctly predicted the presence of both species in the Buffels Bay area but was not able to recover the same result for De Hoop.