Genomic imprinting and communicative behaviour: Prader-Willi and Angelman syndrome

Each cell contains two sets of chromosomes: one set inherited from the mother, the other from the father. Numerical abnormalities such as trisomies result in disease. But uniparental disomies, i.e. two sets of chromosomes from a single parent, may also result in disease if the chromosome harbours imprinted genes. For uniparental disomies lead to imbalance in gene expression. Suppose that, in an offspring, the paternal copy of a gene is normally switched on and the maternal copy is switched off. In the case of a paternal disomy, an offspring would then receive two active copies of the gene while an offspring with maternal disomy would have two inactive copies. Both uniparental disomies can result in disease. Chromosome 15 is an example: a small region of the long arm, namely the 15q11-q13 region, harbours a number of imprinted genes.

Kinship theory has been developed to explain the evolution of behaviour (performed by an actor) that provides a benefit to another individual (the recipient). Such behaviours pose a problem for evolutionary theory, because these behaviours reduce the fitness of the performer of that behaviour. Hamilton argued that behaviours that benefit another can benefit the performer since the two individuals may be related. If they are related, an individual gains inclusive fitness if the performed behaviour has impact on the reproductive success of the related individual. This is called kin selection. Hamilton’s theory demonstrates that cooperative or altruistic behaviour evolve when rb-c>0, where c is the fitness cost of the actor, b is the fitness benefit to the recipient, and r is their genetic relatedness. Hence altruistic or cooperative behaviour evolve if the benefits to the recipient, weighed by the genetic relatedness of the recipient to the actor, outweigh the costs to the actor. The theory predicts greater levels of cooperation when r or b are higher and c is lower. For many relatives r differs for genes of maternal and paternal origin. For that reason a distinction is made between the probability that an individual carries a maternally derived gene (r_m) and a paternally derived gene (r_p). If r_m>r_p (or r_m<r_p), then it is said that there is genetic conflict between paternally and maternally derived genes since an act may enhance the inclusive fitness of maternally derived genes but reduce the inclusive fitness of paternally derived genes (and vice versa).

Imprinted genes are genes that possess, as it were, information about their parental origin. They carry a parental imprint determining whether they will be expressed in offspring. This is achieved by a so-called epigenetic mechanism of gene regulation: certain chemical groups (imprints) are attached to (or removed from) sites on DNA during the formation of the egg and sperm cells. Since these processes depend on the sex of the parent, they result in a parent-specific expression of genes in offspring. If the gene is derived from the father it may be expressed while at the same time the maternal copy is silent (or the other way around). The imprints are removed in the gonads of the children, and afterwards new imprints are established, depending on the sex of the child. There are 100-200 imprinted genes.

A simple calculation shows why human populations started to expand as the result of earlier weaning. Suppose that chimpanzee and human females can conceive children after they are 20 till they are 40. Assuming lactational anoestrus, and that chimpanzees wean at 5 and humans wean at 2.5 years, chimpanzee females can conceive 4 children while human females can conceive 8 children. If 50% die in infancy, only the human population grows.

Some scientists have hypothesised that babbling evolved in the context of cooperative breeding (see for example Hrdy, 2005; Burkart, Fehr, Efferson, Van Schaik, 2007). Hence it is possible that UBE3A influences the development of babbling since babbling optimised the functioning of sharing in the cooperative breeding system and contributes, therefore, to the inclusive fitness of maternally expressed genes.