Ploidy level of genome

So far we have seen that the population dynamic feed-back of interactive competition is selecting for sexual reproduction in multicellular animals. Irrespectively of the size of the interacting unit we found selection for a pair-wise form of sexual reproduction where one half of the genome in the offspring is provided by the mother, and the other half by the father.

While different genetic codes for this sharing can be imagined, the diploid genome, where the female and the male supplies a copy of all the genes in the offspring, is maybe the best solution to avoid selection conflicts to emerge between the genes of the parents. This ploidy level is reflecting the fitness quality of the male, in the sense that the interactive competition of the male is essential for the numerical replication of the genome in the female. We may thus expect that the ploidy level can evolve if the fitness role of the male is changing.

This is likely to be the case in eusocial species, where the direct interactive quality of the male is irrelevant compared with the joint interactive quality of all the offspring workers. The indirect heritable component of the males competitive quality, however, is still needed as it is being selected, by the interactive competition between colonies, to be transferred by sexual reproduction to the offspring workers of the colony.

If the energetics of the sexual male is irrelevant to the colony, he is needed only for mating, as it is the case in eusocial hymenoptera. The two-fold cost of the male is then selecting for a male-haploid female-diploid genome (Fig. 1, right; Witting, 1997, 2007). A symmetrical selection for the diploid genome is instead being maintained if the energetics of the male is essential (Fig. 1, left), and the sexual male and female are forming a lasting bond as in eusocial (and diploid) termites.

Fig. 1 Evolutionary projections for the proportion of female workers and the ploidy level of the genome for eusocial species with (left), and without (right), a pair-bond between the sexual male and female. X-axis: DH:male-diploid female-haploid genome. D:diploid genome. HD:male-haploid female-diploid genome. From Witting (1997).

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References

• Witting, L. 1997. A general theory of evolution. By means of selection by density dependent competitive interactions. Peregrine Publisher, Århus, 330 pp, URL https://mrLife.org.
• Witting, L. 2007. Behavioural interactions selecting for symmetry and asymmetry in sexual reproductive systems of eusocial species. Bulletin of Mathematical Biology 69:1167--1198, https://doi.org/10.1007/s11538--006--9112--x.