Genetic conflict within aphid clones

Joachim L. Dagg 

A genetic conflict exists in aphids. The females of a clone are diploid for the autosomes and sex chromosomes. Parthenogenetic production of males includes the loss of one X chromosome, with no recombination between X chromosomes before the loss. Male offspring is diploid for the autosomes but haploid for the sex chromosomes (XO). Each clone can produce two brotherhoods of males. Those sharing the same X chromosome will be genetically identical. Relatedness between males of the same clone with different X chromosome will be less than 1. Sperm results from meiosis and is haploid, but the sperm cells that do not receive an X chromosome die. All surviving sperm cells share the male’s X chromosome, which makes up the largest part of the genome. A clone’s two brotherhoods of males produce two kinships of sperm. Sperm cells with the same X chromosome will share a part of the clone’s genome that is larger than 0.5. Those carrying different X chromosomes will share less than 0.5. 

This changes the default expectation for aphids from no to widespread sperm competition. The inconclusive evidence on adaptations towards sperm competition in aphids is, therefore, unexpected and needs an explanation. 

The genetic conflict plays out at the level of genes, sperm, and behavior. Non-Mendelian inheritance of X chromosomes to male offspring can be a genetic resolution of the conflict. If genetic resolutions were widespread among aphids, this could explain the surprising scarcity of adaptations to the conflict at the level of male behavior or sperm.