Marine Hybrid Zone Case Study
Hybrid zones are thought to be less common in marine than terrestrial environments. However, blue mussel populations show extensive hybridisation worldwide and are a well studied example of a marine hybrid zone. There are multiple sites of hybridisation between the closely related species Mytilus edulis, M. trossulus and M. galloprovincialis across the North Atlantic and Pacific coasts. These hybrid zones vary considerably. Some hybrid zones, such as the one in Newfoundland in Canada show remarkably few hybrids, while in the Baltic Sea most individuals are hybrids.
Based on the fossil record and genetic marker studies the following chronology is used to explain the Canadian mussel hybrid zone:
- The genus Mytilus is at one point restricted to the North Pacific but spreads to the Atlantic through the Bering Strait around 3.5 million years ago.
- M. trossulus evolves in the North Pacific and M. edulis in the Atlantic in near allopatry as migration across the Bering Strait is very low.
- Recently, in post-glacial time M. trossulus from the Pacific enters the Atlantic and colonises shores on both sides, and meets with the local M. edulis..
The Canadian mussel hybrid zone is unusual because both species are found along the entire shore (a mosaic pattern) instead of the typical cline found in most hybrid zones. Studies of mtDNA and allozymes in adult populations show that the distribution of genotypes between the two species is bimodal; pure parental types are most common (representing above 75% of individuals) while backcrosses close to parental forms are the next most prevalent. F1 hybrid crosses represent less than 2.5% of individuals.
The low frequency of F1 hybrids coupled with some introgression allows us to infer that although fertile hybrids can be produced, significant reproductive barriers exist and the two species are sufficiently deviated that they are now able to avoid recombinational collapse despite habitat sharing. One reason that could account for keeping taxa separate through prezygotic isolation is that in this region M. edulis spawns over a narrow 2–3 week period in July, while M. trossulus spawned over a more extensive period between late spring to early autumn. No infertility or developmental retardation was found in the hybrid indiviuals, allowing them to introgress with pure species.
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