My post yesterday offered up a quick sketch of the phenomenon of genomic imprinting. In short, genomic imprinting is the selective expression of an allele conditional upon whether it is inherited from the father or mother. This selective expression is limited to a small subset of loci, perhaps about 200 in the typical mammal. These expression patterns often relate to conflicts over resources between offspring and mother, and have fitness implications for all individuals in question, mother, father and offspring. All of this is derived from the initial logic that maternal and paternal copies of alleles might have different interests in regards to resource allocation between mother and offspring. This makes sense, doesn’t it? You have parents, and your father wants you to suceed at all costs, no matter the harm to your mother. Wait you say, back up, something doesn’t sound right. And yes, I left something out….
Asymmetrical expression implies deviation from Ozzie & Harriet monogamy and equal investment. Obviously for a monogamous male the female who gives birth to one litter or offspring is not simply a vessel that may be discarded, that female is the mother of future offspring, and so is critically tied to his own fitness. On the previous post I offered the example of paternal vs. maternal half-siblings to show how imprinting dynamics could work. In most mammals it is the mother who invests the most into parental care, and the offspring are generally socialized early on amongst litter-mates in the presence of a common mother. On the other hand, they may have different fathers, even within the same litter because of multiple paternity. Double paternity is known to occur in human twins as well, and it is not uncommon amongst creatures that are more polyandrous than our own. Because maternal copies are more likely to be shared than paternal copies you have conflicting interests within the nest. Paternal copies are extremely selfish, while maternal copies are more likely to want you to behave in an altruistic fashion because of inclusive fitness.
So, does this really mean anything in the real world? First, let’s get the simplest first order explanations down. Consider:
A) Species Polyandry (SP)
B) Species Monogamy (SM)
These two species are very similar, sister species of the same genus, but they are differentiated according to this behavioral trait. In the latter species pair-bonding is crucial, male investment is expected and incidence of multiple paternity within a litter is low. In the former species there is no post-coital pair-bonding, male investment ends at the sperm and incidence of multiple paternity is high. What should we expect? In SP the males would be predicted to exhibit “selfish” hyper-expressing growth genes. Since all they invest is sperm, and they are not likely to mate again with the same female, these males don’t particularly care if the females are in shape for future gestations. In response the females should develop counter-responses, repression to mask some of the over-activity of the paternal locus. In short, a classic evolutionary game of attack and response, ratcheting upward along stringes of macromolecular interactions.
In contrast to SP, one would expect little from SM. One could expect biallelic expression, but the key is that male and female copies of alleles would have similar interests. If the male copy induced growth, it would be hurting its own future fitness because of how closely tied it is with the maternal copies (in the form of the bonded female). This does not negate the possibility of parent-offspring conflict, but in this case the issues would be narrower and more constrained since the parents share interests.
Now, to a concrete case. Peromyscus maniculatus and Peromyscus polionotus are sister species, but while the former is polyandrous, the latter is monogamous. When you cross these two species, the F1 generation varies a great deal in size. P. polionotus females bred with P. maniculatus males give birth to enormous offspring, while P. maniculatus females whose offspringed are sired by P. polionotus males give birth to small offspring. The reason is clear, the polyandrous species has powerful imprinting mechanisms, and the over-expression and repression swamp the genetic pathways of the monogamous species.
Polyandry vs. monogamy is just the beginning. In future posts I will explore some of the weird diseases and social evolutionary implications of these 200 or so imprinted loci.
Related: Kinship & imprinting.
Burt et. al., Genes in Conflict, 2005
Wilkins et. al., What Good is Genomic Imprinting: the function of parent-specific gene expression, Nat Rev Genet. 2003 May;4(5):359-68