Hybrid zones, barrier loci and the ‘rare allele phenomenon’

Lammers, Youri

doi:10.1111/jeb.12056

Cited by 7 publications

(7 citation statements)

References 19 publications

(22 reference statements)

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“…The existence of Ea and Ef hybridization make these common species easily maintained in laboratory the attractive models for studies on mechanisms of interspecies gene flow during introgressive hybridization [ 46 , 47 ], phenomenon described in other lumbricids, Allolobophora sp. [ 48 ] and Lumbricus sp.…”

Section: Discussionmentioning

confidence: 99%

The existence of fertile hybrids of closely related model earthworm species, Eisenia andrei and E. fetida

et al. 2018

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Lumbricid earthworms Eisenia andrei (Ea) and E. fetida (Ef) are simultaneous hermaphrodites with reciprocal insemination capable of self-fertilization while the existence of hybridization of these two species was still debatable. During the present investigation fertile hybrids of Ea and Ef were detected. Virgin specimens of Ea and Ef were laboratory crossed (Ea+Ef) and their progeny was doubly identified. 1 –identified by species-specific maternally derived haploid mitochondrial DNA sequences of the COI gene being either ‘a’ for worms hatched from Ea ova or ‘f’ for worms hatched from Ef ova. 2 –identified by the diploid maternal/paternal nuclear DNA sequences of 28s rRNA gene being either ‘AA’ for Ea, ‘FF’ for Ef, or AF/FA for their hybrids derived either from the ‘aA’ or ‘fF’ ova, respectively. Among offspring of Ea+Ef pairs in F1 generation there were mainly aAA and fFF earthworms resulted from the facilitated self-fertilization and some aAF hybrids from aA ova but none fFA hybrids from fF ova. In F2 generation resulting from aAF hybrids mated with aAA a new generations of aAA and aAF hybrids were noticed, while aAF hybrids mated with fFF gave fFF and both aAF and fFA hybrids. Hybrids intercrossed together produced plenty of cocoons but no hatchlings independently whether aAF+aAF or aAF+fFA were mated. These results indicated that Ea and Ef species, easy to maintain in laboratory and commonly used as convenient models in biomedicine and ecotoxicology, may also serve in studies on molecular basis of interspecific barriers and mechanisms of introgression and speciation. Hypothetically, their asymmetrical hybridization can be modified by some external factors.

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Section: Discussionmentioning

confidence: 99%

The existence of fertile hybrids of closely related model earthworm species, Eisenia andrei and E. fetida

et al. 2018

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“…We propose that the optimal K value of 3 implies the virtual existence of a hybrid zone. Natural hybrid zones have been observed in the distribution of many plant species [8], [9], [35] and play important roles in intermediating gene introgression between parental populations [7]. At K = 3, intensive backcrossing was inferred to occur in both directions, with an indistinct genetic boundary associated with the hybrid zone due to intensive introgression with both parental species.…”

Section: Resultsmentioning

confidence: 99%

“…On the other hand, various intrinsic or extrinsic reproductive barriers that reduce hybrid fitness can be formed through introgressive hybridization, thus contributing to the maintenance of species integrity [6]. A balance between gene flow and hybrid barriers is believed to maintain the hybrid zones [7], [8] that develop when hybridization occurs between species with different environmental adaptations [9]. Environmental heterogeneity can lead to a ‘mosaic’ structure in the hybrid zone [10]–[12], as genotype selection often depends on habitat attributes.…”

Section: Introductionmentioning

confidence: 99%

Genetic Introgression and Species Boundary of Two Geographically Overlapping Pine Species Revealed by Molecular Markers

et al. 2014

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Gene introgression and hybrid barriers have long been a major focus of studies of geographically overlapping species. Two pine species, Pinus massoniana and P. hwangshanensis, are frequently observed growing adjacent to each other, where they overlap in a narrow hybrid zone. As a consequence, these species constitute an ideal system for studying genetic introgression and reproductive barriers between naturally hybridizing, adjacently distributed species. In this study, we sampled 270 pine trees along an elevation gradient in Anhui Province, China and analyzed these samples using EST-SSR markers. The molecular data revealed that direct gene flow between the two species was fairly low, and that the majority of gene introgression was intermediated by backcrossing. On the basis of empirical observation, the on-site distribution of pines was divided into a P. massoniana zone, a hybrid zone, and a P. hwangshanensis zone. STRUCTURE analysis revealed the existence of a distinct species boundary between the two pine species. The genetic boundary of the hybrid zone, on the other hand, was indistinct owing to intensive backcrossing with parental species. Compared with P. massoniana, P. hwangshanensis was found to backcross with the hybrids more intensively, consistent with the observation that morphological and anatomical characteristics of trees in the contact zone were biased towards P. hwangshanensis. The introgression ability of amplified alleles varied across species, with some being completely blocked from interspecific introgression. Our study has provided a living example to help explain the persistence of adjacently distributed species coexisting with their interfertile hybrids.

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“…For example, selection could disfavour alleles that contribute to deleterious BDMs or selection could favour alleles that modify and ameliorate BDMs. The resulting decline in frequency of deleterious alleles (or increase in frequency of modifier alleles) would reduce the adverse fitness effects of hybridization and hybrids could become more similar to the pure‐species types over time (Barton & Hewitt, , ; Lammers et al, ; Ritchie, Butlin, & Hewitt, ; Sanderson, ; Schilthuizen & Lammers, ). Regardless of how BDMs evolve (via selection or otherwise), change in BDMs will impact the nature of introgression between species and the maintenance of species boundaries.…”

Section: Introductionmentioning

confidence: 99%

Variation in hybrid gene expression: Implications for the evolution of genetic incompatibilities in interbreeding species

et al. 2019

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Interbreeding species often produce low‐fitness hybrids due to genetic incompatibilities between parental genomes. Whether these incompatibilities reflect fixed allelic differences between hybridizing species, or, alternatively, standing variants that segregate within them, remains unknown for many natural systems. Yet, evaluating these alternatives is important for understanding the origins and nature of species boundaries. We examined these alternatives using spadefoot toads (genus Spea), which naturally hybridize. Specifically, we contrasted patterns of gene expression in hybrids relative to pure‐species types in experimentally produced tadpoles from allopatric parents versus those from sympatric parents. We evaluated the prediction that segregating variation should result in gene expression differences between hybrids derived from sympatric parents versus hybrids derived from allopatric parents, and found that 24% of the transcriptome showed such differences. Our results further suggest that gene expression in hybrids has evolved in sympatry owing to evolutionary pressures associated with ongoing hybridization. Although we did not measure hybrid incompatibilities directly, we discuss the implications of our findings for understanding the nature of hybrid incompatibilities, how they might vary across populations over time, and the resulting effects on the evolutionary maintenance ‐ or breakdown ‐ of reproductive barriers between species.

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Hybrid zones, barrier loci and the ‘rare allele phenomenon’

Cited by 7 publications

References 19 publications

The existence of fertile hybrids of closely related model earthworm species, Eisenia andrei and E. fetida

The existence of fertile hybrids of closely related model earthworm species, Eisenia andrei and E. fetida

Genetic Introgression and Species Boundary of Two Geographically Overlapping Pine Species Revealed by Molecular Markers

Variation in hybrid gene expression: Implications for the evolution of genetic incompatibilities in interbreeding species

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