Hybrid zone

A hybrid zone exists where the ranges of two interbreeding species meet. For a hybrid zone to be stable, the offspring produced by the cross (the hybrids) have to be less fit than members of the parent species, although this condition does not need to be met in the very first hybrid generation (F1 hybrid which can exhibit hybrid vigour). Some hybrid zones move, typically at a rate of 0.1-10 metres per year.

Hybrid zones are relatively rare, although a surprising number are now known to science. They present a problem to the taxonomy of the species involved, and the definition of species more generally. They are also important study systems for understanding how new species form (Hybrid speciation), as they are believed to be in transition to reproductive isolation.

These zones are often mapped including the current range of both species, with the overlap ranges highlighted.

Hybrid zones are locations where the hybrid offspring of two divergent populations (sometimes defined as subspecies or races) are prevalent and form a cline (Barton & Hewitt, 1985). Precise definitions of hybrid zones vary, some insist on increased variability of fitness within the zone, others that hybrids be identifiably different from parental forms and others that they represent secondary contact alone (Murray, 1985). They occur at the area of contact between two closely related but genetically different populations, each regarded as parental forms. Reviews of hybrid zones show varying widths between hundreds of metres and hundreds of kilometres and the presence of both gradual clines and stepped clines (Barton & Hewitt, 1985). They present a paradox for the biological definition of a species, usually “a population of actually or potentially interbreeding individuals that produce fertile offspring” (Mayr, 1942). Both parental forms by this definition are one species as they can produce fertile offspring at least some of the time. Despite this, the two populations remain identifiably different, conforming to an alternative definition of species as “taxa that retain their identity despite gene flow” (Barton and Hewitt, 1989). They are useful in the study of the processes of speciation as they provide natural examples of gene flow between populations that are at some point between representing a single species and representing multiple species in reproductive isolation.

The clines of hybrid zones can be observed by recording the frequency of certain diagnostic alleles or phenotypic characteristics for either population along a transect between the two populations. They almost always take the form of a sigmoid curve. They can be wide (gradual) or narrow (steep) depending on the ratio of hybrid survival to recombination of genes (Barton, 1983).

One form of hybrid zone results where one species has undergone allopatric speciation and the two new populations regain contact after a period of geographic isolation. The two populations then mate within an area of contact, producing 'hybrids' which contain a mixture of the alleles distinctive for each population. Thus novel genes flow from either side into the hybrid zone. Genes can also flow back into the distinct populations through interbreeding between hybrids and parental (non-hybrid) individuals (introgression) (Ridley 2003). These processes lead to the formation of a cline between the two pure forms within the hybrid zone.

Hybrid zones and gene flow do not inevitably lead to the recombination of the two populations involved, but can instead continue for thousands of years (White, et al, 1967.). The predominant explanation for this is that the hybrid zone represents a 'tension zone' between the conflicting effects of dispersal of parental forms and selection against hybrids (Bazykin, 1969.). Dispersal of individual parents leads to the creation of more hybrids within the hybrid zone. This results in gene flow between the two populations because of introgression. However, in many cases hybrids are less fit than parental forms because they lack the complete gene complexes of the parentals that make them well adapted to the environments either side of the hybrid zone. The more frequent death and sterility of hybrids forms a barrier to gene flow by making a 'hybrid sink' into which genes from parentals flow but rarely continue into the other population. Statistical models suggest that neutral alleles flow across this barrier very slowly while positively selected alleles will move across quite rapidly (Barton & Hewitt, 1985 p.135). An interesting outcome of this model is that hybrid zones are almost environment independent and can therefore move (Barton, 1979). Hybrids may not always be unfit in the very first generation, which can show hybrid vigour.

Several other models exist to explain hybrid zone stability, although the tension zone model is used in most cases. The dispersal-independent cline model does not consider dispersal at all, with the frequency of alleles finding different equilibria depending on the precise environmental conditions in a particular area. In each location, selection maintains a stable equilibria for each allele, resulting in a smooth cline. (Moore, 1977) The hybrids must therefore be fitter at some point along the cline. The wave of advance model sees multiple clines for individual alleles forming due to the progression of advantageous alleles from one population the other (Pialek and Barton, 1997).

Certain factors contribute to stability and steepness of hybrid zones within these models by reducing the frequency of inter-population mating and introgression. These include positive assortative mating within populations, habitat selection of different populations (examples of both these found in question 1 part B and question 2) and hybrid unfitness. Additionally, it is suggested that individuals in a populations near a tension zone (in which hybrids are less fit), will evolve methods of only mating with their own population to reduce the prevalence of unfit hybrids. This is dubbed reinforcement, and controversy remains as to its importance (Howard, 1993).

Hybrid zones can also occur across regions of primary contact in which parapatric speciation is taking place. As a population spreads across a contiguous area it may spread into an abruptly different environment. The populations will deviate and begin parapatric speciation – those in the new environment adapting to the different conditions. The point of contact between the older population and the newer population will be a stepped cline and a hybridisation zone can form. Despite this, the two populations will never have been fully isolated from one another, unlike in cases of secondary contact. This distinction may not be a very useful one as in practice it can be quite difficult to distinguish between primary and secondary contact (Endler, 1982).

Hybrid Zone Case Study

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 species Mytilus edulis, M. trossulus and M. galloprovincialis across the Atlantic, Scandinavian and the Mediterranean Seas. These hybrid zones vary considerably. Some hybrid zones, such as the one in Newfoundland in Canada show remarkably few hybrids, while in Scandinavia most individuals are hybrids to some degree. Morphological and genetic differences are clear between these populations and it is believed that they are close to full speciation. They are probably maintained through a combination of hybrid unfitness, positive assortative mating and habitat segregation. In this summary I will focus on the Canadian hybrid zone in the North Atlantic, particularly that near Newfoundland which has been studied by researchers at Newfoundland Memorial University.

Based on the fossil record and genetic marker studies the following chronology is used to explain the Mussel hybrid zone:
*The genus Mytilus is at one point restricted to the North Pacific but spreads to the Atlantic through the Bering Straight around 3.5 million years ago (Vermeij 1991).
*M. trossulus evolves in the North Pacific and M. edulis in the Atlantic in near allopatry as migration across the Bering Straight is very low.
*Subsequently M. galloprovincialis undergoes cladogenesis from M. edulis in the Mediterranean Sea after it is temporarily isolated from the Atlantic.
*Recently M. trossulus from the Pacific enters the Atlantic and colonises shores on both sides. It spreads and forms secondary contact hybrid zones with M. edulis populations on coasts across Scandinavia and the eastern Atlantic.
*Riginos and Cunningham (2005) includes a suggested pattern of migration of M. trossulus across the ocean based on a review of genetic marker studies.

The Canadian 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 (Bates and Innes, 1995). 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 (Saavedra et al., 1996).

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. Jiggins and Mallet (2000) have found that hybrid zones with low levels of F1 hybrids (bimodal distribution of hybrids) are highly stable and usually the result of assortative mating or fertilization, and not related to gross levels of genetic divergence or intrinsic genomic incompatibility. They hypothesized that such zones occurred between two populations that were close to full speciation and with some reinforcement. Toro et al (2002) investigated whether different reproductive patterns and behaviour were the cause of this prezygotic isolation and discovered that M. edulis spawned over a 2-3 week period in July, while M. trossulus spawned over a more extensive period between late spring to early autumn. It was also found that hybrids were not infertile and exhibited normal reproductive development, allowing them to introgress with pure species. It was concluded that “differences in reproductive traits may partially explain the maintenance of the mussel hybrid zone in Newfoundland.”

The other likely candidate for hybrid zones stability is species segregation by habitat which has been investigated but not conclusively. Several studies have suggested that M. edulis are found in areas of lower salinity and less wave exposure at the heads of bays more than M. trosullus. M.trosullus appears to be favoured in habitats with higher wave exposure (Bates and Innes, 1995). The one subtidal (low wave action) site sampled by Bates and Innes had just 8% M. trossulus individuals. A similar segregation has been found in the Mediterranean hybrid zone with M. edulis also favouring more sheltered habitats compared to M. galloprovincialis (Bierne et al. 2003). If this is the case, this would provide partial habitat separation and reduce the probability of gametes of two species encountering one another and cross-fertilising. This would increase genetic distinctiveness despite the populations living in sympatry. However, conflicting results have been identified to this trend of habitat segregation and so these results are not conclusive (Riginos and Cunningham, 2005). It is suggested that differences in habitat are what has led to the very different type of hybrid zones in Scandinavia and Canada. Hybrid mussel fitness has not been properly investigated, so it is not possible to judge its effects on postzygotic isolation and whether it could cause reinforcement (Riginos and Cunningham, 2005).

ee also

* Genetic pollution

References

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