The hidden threat: genetic load dynamics in tetraploids and diploids.

Polyploids are organisms with three or more chromosome sets, which often give them different physiology, morphology, and evolutionary properties. The impact of polyploidy on the genetic load dynamics, in terms of accumulation, fixation, and purging of deleterious mutations, is largely unknown. Here, we use forward-in-time genomic simulations to compare the fitness effects of genetic load components assuming diploid and tetraploid populations under different demographic scenarios. Under partial recessiveness, we show that tetraploids tend to accumulate a higher genetic load than diploids in small and large populations, with negative consequences on their fitness. Under complete recessiveness, the same pattern is observed only in small populations, but similar load effects are found in large diploid and tetraploid populations under mutation-drift equilibrium. The only scenario where tetraploids suffered less from the effects of genetic load was in bottlenecked populations under complete recessiveness. Our results highlight the importance of factors such as demography, dominance and selection coefficients, and genetic drift in shaping the different patterns of accumulation of deleterious mutations in diploids and tetraploids. While some studies have suggested that tetraploids might have a reduced genetic load because recessive mutations could be masked more efficiently, our findings align more closely with evidence that tetraploids frequently accumulate a higher genetic load than diploids. This insight is significant for assessing the role of ploidy in the conservation of endangered species.