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Reinforcement of genetic coherence in a two-locus model.

Gregorius HR, Steiner W - BMC Evol. Biol. (2001)

Bottom Line: Since heterozygote advantage at the object locus is a necessary condition for this result to hold true, reinforcement of genetic coherence can be claimed for this case.To allow for a generalized analysis including alternative models of genetic coherence as well as the estimation of its degrees in real populations, an operational concept for the measurement of this degree is developed.The resulting index is applied to the interpretation of data from crossing experiments in Alnus species designed to detect incompatibility relations.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institut fur Forstgenetik und Forstpflanzenzüchtung Universität Göttingen, Büsgenweg 2, D-37077 Göttingen, Germany. gregorius@gwdg.de

ABSTRACT

Background: In order to maintain populations as units of reproduction and thus enable anagenetic evolution, genetic factors must exist which prevent continuing reproductive separation or enhance reproductive contact. This evolutionary principle is called genetic coherence and it marks the often ignored counterpart of cladistic evolution. Possibilities of the evolution of genetic coherence are studied with the help of a two-locus model with two alleles at each locus. The locus at which viability selection takes place is also the one that controls the fusion of gametes. The second locus acts on the first by modifying the control of the fusion probabilities. It thus acts as a mating modifier whereas the first locus plays the role of the object of selection and mating. Genetic coherence is enhanced by modifications which confer higher probabilities of fusion to heterotypic gametic combinations (resulting in heterozygous zygotes) at the object locus.

Results: It is shown that mutants at the mating modifier locus, which increase heterotypic fusions but do not lower the homotpyic fusions relative to the resident allele at the object locus, generally replace the resident allele. Since heterozygote advantage at the object locus is a necessary condition for this result to hold true, reinforcement of genetic coherence can be claimed for this case. If the homotypic fusions are lowered, complex situations may arise which may favor or disfavor the mutant depending on initial frequencies and recombination rates. To allow for a generalized analysis including alternative models of genetic coherence as well as the estimation of its degrees in real populations, an operational concept for the measurement of this degree is developed. The resulting index is applied to the interpretation of data from crossing experiments in Alnus species designed to detect incompatibility relations.

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Schematic representation of the model determinants of reproduction and survival: (1) gametes encounter at random, (2) gamete fusion is determined by the B-locus (the object locus of mating), and fusion probabilities are modified by the A-locus (the mating modifier locus), (3) survival is determined by the B-locus (the object locus of survival) only; the B-locus thus is object of both gamete fusion and survival. The framed part refers to differential fusion modification with respect to homotypic and heterotypic encounters at the object locus B.
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Figure 1: Schematic representation of the model determinants of reproduction and survival: (1) gametes encounter at random, (2) gamete fusion is determined by the B-locus (the object locus of mating), and fusion probabilities are modified by the A-locus (the mating modifier locus), (3) survival is determined by the B-locus (the object locus of survival) only; the B-locus thus is object of both gamete fusion and survival. The framed part refers to differential fusion modification with respect to homotypic and heterotypic encounters at the object locus B.

Mentions: A model with two biallelic loci A and B is considered in which viability selection (in the diplophase) is restricted to the genotypes at the B-locus. The B-locus is also involved in the formation of zygotes in that the alleles present at this locus in the encountering gametes of different sex specificity determine their probabilities of fusion. The role of the A-locus consists solely in modifying these fusion probabilities according to the alleles carried by the encountering gametes at this locus. No selection occurs at the A-locus. As suggested by [4], the A- and B-locus will be referred to as the mating modifier and the object locus, respectively. Gametes of different sex specificity are assumed to encounter at random. The loci are linked with recombination rate r (the relevant notational details are compiled in Table 1 and illustrated in Figure 1). Generations are assumed to be separated and population size is effectively infinite.


Reinforcement of genetic coherence in a two-locus model.

Gregorius HR, Steiner W - BMC Evol. Biol. (2001)

Schematic representation of the model determinants of reproduction and survival: (1) gametes encounter at random, (2) gamete fusion is determined by the B-locus (the object locus of mating), and fusion probabilities are modified by the A-locus (the mating modifier locus), (3) survival is determined by the B-locus (the object locus of survival) only; the B-locus thus is object of both gamete fusion and survival. The framed part refers to differential fusion modification with respect to homotypic and heterotypic encounters at the object locus B.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC58585&req=5

Figure 1: Schematic representation of the model determinants of reproduction and survival: (1) gametes encounter at random, (2) gamete fusion is determined by the B-locus (the object locus of mating), and fusion probabilities are modified by the A-locus (the mating modifier locus), (3) survival is determined by the B-locus (the object locus of survival) only; the B-locus thus is object of both gamete fusion and survival. The framed part refers to differential fusion modification with respect to homotypic and heterotypic encounters at the object locus B.
Mentions: A model with two biallelic loci A and B is considered in which viability selection (in the diplophase) is restricted to the genotypes at the B-locus. The B-locus is also involved in the formation of zygotes in that the alleles present at this locus in the encountering gametes of different sex specificity determine their probabilities of fusion. The role of the A-locus consists solely in modifying these fusion probabilities according to the alleles carried by the encountering gametes at this locus. No selection occurs at the A-locus. As suggested by [4], the A- and B-locus will be referred to as the mating modifier and the object locus, respectively. Gametes of different sex specificity are assumed to encounter at random. The loci are linked with recombination rate r (the relevant notational details are compiled in Table 1 and illustrated in Figure 1). Generations are assumed to be separated and population size is effectively infinite.

Bottom Line: Since heterozygote advantage at the object locus is a necessary condition for this result to hold true, reinforcement of genetic coherence can be claimed for this case.To allow for a generalized analysis including alternative models of genetic coherence as well as the estimation of its degrees in real populations, an operational concept for the measurement of this degree is developed.The resulting index is applied to the interpretation of data from crossing experiments in Alnus species designed to detect incompatibility relations.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institut fur Forstgenetik und Forstpflanzenzüchtung Universität Göttingen, Büsgenweg 2, D-37077 Göttingen, Germany. gregorius@gwdg.de

ABSTRACT

Background: In order to maintain populations as units of reproduction and thus enable anagenetic evolution, genetic factors must exist which prevent continuing reproductive separation or enhance reproductive contact. This evolutionary principle is called genetic coherence and it marks the often ignored counterpart of cladistic evolution. Possibilities of the evolution of genetic coherence are studied with the help of a two-locus model with two alleles at each locus. The locus at which viability selection takes place is also the one that controls the fusion of gametes. The second locus acts on the first by modifying the control of the fusion probabilities. It thus acts as a mating modifier whereas the first locus plays the role of the object of selection and mating. Genetic coherence is enhanced by modifications which confer higher probabilities of fusion to heterotypic gametic combinations (resulting in heterozygous zygotes) at the object locus.

Results: It is shown that mutants at the mating modifier locus, which increase heterotypic fusions but do not lower the homotpyic fusions relative to the resident allele at the object locus, generally replace the resident allele. Since heterozygote advantage at the object locus is a necessary condition for this result to hold true, reinforcement of genetic coherence can be claimed for this case. If the homotypic fusions are lowered, complex situations may arise which may favor or disfavor the mutant depending on initial frequencies and recombination rates. To allow for a generalized analysis including alternative models of genetic coherence as well as the estimation of its degrees in real populations, an operational concept for the measurement of this degree is developed. The resulting index is applied to the interpretation of data from crossing experiments in Alnus species designed to detect incompatibility relations.

Show MeSH
Related in: MedlinePlus