Genomic imprinting has been proposed to evolve when a gene's expression has fitness consequences for individuals with different coefficients of matrilineal and patrilineal relatedness, especially in the context of competition between offspring for maternal resources. Previous models have focused on pre-emptive hierarchies, where conflict arises with respect to resource allocation between present and future offspring. Here we present a model in which imprinting arises from scramble competition within litters. The model predicts paternal-specific expression of a gene that increases an offspring's fractional share of resources but reduces the size of the resource pool, and maternal-specific expression of a gene with opposite effects. These predictions parallel the observation in economic models that individuals tend to underprovide public goods, and that the magnitude of this shortfall increases with the number of individuals in the group. Maternally derived alleles are more willing than their paternally derived counterparts to contribute to public goods because they have a smaller effective group size.
Comparative gene mapping and chromosome painting permit the tentative reconstruction of ancestral karyotypes. The modern human karyotype is proposed to differ from that of the most recent common ancestor of catarrhine primates by two major rearrangements. The first was the fission of an ancestral chromosome to produce the homologues of human chromosomes 14 and 15. This fission occurred before the divergence of gibbons from humans and other apes. The second was the fusion of two ancestral chromosomes to form human chromosome 2. This fusion occurred after the divergence of humans and chimpanzees. Moving further back in time, homologues of human chromosomes 3 and 21 were formed by the fission of an ancestral linkage group that combined loci of both human chromosomes, whereas homologues of human chromosomes 12 and 22 were formed by a reciprocal translocation between two ancestral chromosomes. Both events occurred at some time after our most recent common ancestor with lemurs. Less direct evidence suggests that the short and long arms of human chromosomes 8, 16 and 19 were unlinked in this ancestor. Finally, the most recent common ancestor of primates and artiodactyls is proposed to have possessed a chromosome that combined loci from human chromosomes 4 and 8p, a chromosome that combined loci from human chromosomes 16q and 19q, and a chromosome that combined loci from human chromosomes 2p and 20.
Callitrichid primates typically give birth to twin offspring that are somatic chimeras of cells derived from two products of conception. Each individual is thus the phenotype of two sibling genotypes, one of which may be more closely related to the germ line of the individual's parents than to the individual's own germ line. Chimerism could therefore help to explain the evolution of alloparental care and social suppression of reproduction in callitrichids. Placental chimerism may also have important implications for understanding kin interactions within the womb: on one side of the coin, the intimate juxtaposition of genotypes provides unique opportunities for antagonistic interactions between embryos; on the other side, chimerism could facilitate cooperation between sibling genotypes.
The Evolution of Sibling Rivalry by D.W. Mock and G.A. Parker Oxford University Press, 1998. $120.00 hbk, $55.00 pbk (xiii +464 pages) ISBN 0 19 857743 5/0 19 857744 3.
Two models of maternal-fetal interactions are discussed. In the first, offspring are advantaged if they possess an allele absent in their mother. Polymorphism is maintained because rare alleles have an advantage when present in males. In the second, offspring are disadvantaged if they lack an allele present in their mother. Polymorphism is maintained because rare alleles have an advantage when present in females. Both classes of model are associated with a deficiency of homozygous genotypes. If the artificial assumption of symmetrical selection is relaxed, the second class of model (gestational drive) could account for the otherwise inexplicable absence of MHC polymorphism in some species.
The theory of inclusive fitness can be modified to consider separate coefficients of relatedness for an individual's maternal and paternal alleles. A gene is said to have parentally antagonistic effects if it has an inclusive fitness benefit when maternally derived, but an inclusive fitness cost when paternally derived (or vice versa). Parental antagonism favours the evolution of alleles that are expressed only when maternally derived or only when paternally derived (genomic imprinting).
Pregnancy is traditionally viewed as a harmonious collaboration between mother and fetus. From this perspective, viviparity poses a series of problems that maternal and fetal genes work together to solve and the many complications of pregnancy are interpreted as evidence of the malfunctioning of an evolved system or of the failure of natural selection to achieve an adaptive goal. This view fails to recognize aspects of genetic conflict that lie at the heart of gestation. At least three interrelated sources of conflict can be identified: (i) conflict between genes expressed in the mother and genes expressed in the fetus/placenta (parent-offspring conflict); (ii) conflict between maternally-derived and paternally-derived genes within the fetal genome (genomic imprinting); and (iii) conflict between maternal genes that recognize themselves in offspring and the rest of the maternal genome (gestational drive).
A gene is described as imprinted if its pattern of expression depends on whether it passed the previous generation in a male or female germ line. A recent paper reports that imprinted genes have fewer and smaller introns than a control set of genes. The differences are striking but their interpretation is unclear. The loss of introns after a gene becomes imprinted is not sufficient to explain why imprinted genes have fewer introns than average, because related unimprinted genes also have few introns. Similarly, small introns appear to be a property of chromosomal region rather than of imprinting status itself, because neighboring unimprinted genes also have small introns.
A "green beard" refers to a gene, or group of genes, that is able to recognize itself in other individuals and direct benefits to these individuals. Green-beard effects have been dismissed as implausible by authors who have implicitly assumed sophisticated mechanisms of perception and complex behavioral responses. However, many simple mechanisms for genes to "recognize" themselves exist at the maternal-fetal interface of viviparous organisms. Homophilic cell adhesion molecules, for example, are able to interact with copies of themselves on other cells. Thus, the necessary components of a green-beard effect -- feature, recognition, and response -- can be different aspects of the phenotype of a single gene. Other green-beard effects could involve coalitions of genes at closely linked loci. In fact, any form of epistasis between a locus expressed in a mother and a closely linked locus expressed in the fetus has the property of "self-recognition." Green-beard effects have many formal similarities to systems of meiotic drive and, like them, can be a source of intragenomic conflict.
