Reversal of Dominance

 

 

 It has been known for some time that hybrids are usually not exactly intermediate between the parental species. Mendel introduced the term dominant to refer the characters that are transmitted almost unchanged to the hybrids and recessive to those that become latent. Dominance is a ubiquitous phenomenon with most deleterious mutations being recessive. R.A. Fisher (1928) raised the question that, why most deleterious mutations are recessive and proposed a model for the evolutionary modification of dominance relations. Fisher's theory was criticized by Sewall Wright who gave explanation of dominance has strong physiological component. Despite its central importance and a huge body of empirical and theoretical work, no general valid explanation for dominance has been provided until today.¹

Under antagonistic selection, beneficial reversal of dominance can greatly increase the magnitude of marginal over dominance (i.e., by increasing the mean fitness of heterozygote). Beneficial reversal of dominance is a specific case of such variation in dominance, where an allele that is beneficial for some traits and detrimental for other traits is always dominant in the beneficial and recessive in the detrimental, respectively.

      In general, the extent of dominance is not a fixed value, but can vary as a function of genetic background or environmental factors¹. It is theoretically possible that plasticity in dominance could be selected for, to produce higher dominance of conditionally beneficial alleles for the conditions under which they are beneficial.¹

Through beneficial reversal of dominance, invasions into novel environments would be facilitated, as successful invasion often requires very rapid adaptation. Such adaptation is more likely to occur from standing genetic variation rather than from de novo beneficial mutations arising during invasion events.³Reversal of dominance might also increase initial rates of survival and reduce the risk of population extinction upon introduction into a new environment.³Many successful studies have not been carried out in plants that unequivocally give the proof of dominance reversal for a gene of economic interest but that creates ample scope to direct research in this direction. Future studies can focus on the role of dominance reversal on introduction of a new genotype, photoperiod insensititvity, identification of critical range for best performance and variation in a trait by varying genetic backgrounds.

References:

 

1.      BOURGUET, D., 1999, The evolution of dominance. Heredity., 83:1–4.

 

2.      FISHER, R. A., 1928, The possible modification of the response of the wild type to recurrent mutations. American Naturalist, 62:115–126.

 

3.      POSAVI, M., GELEMBIUK, G. W., LARGET, B. AND LEE, C. E., 2014, Testing for beneficial reversal of dominance during salinity shifts in the invasive copepod Eurytemora affinis, and implications for the maintenance of genetic variation. Evolution., 68: 3166–3183.