Abstract
Maternally inherited bacterial endosymbionts that affect host fitness are common in nature. Some endosymbionts colonise host populations by reproductive manipulations (such as cytoplasmic incompatibility; CI) that increase the reproductive fitness of infected over uninfected females. Theory predicts that CI-inducing endosymbionts in haplodiploid hosts may also influence sex allocation, including in compatible crosses, however, empirical evidence for this is scarce. We examined the role of two common CI-inducing endosymbionts, Cardinium and Wolbachia, in the sex allocation of Pezothrips kellyanus, a haplodiploid thrips species with a split sex ratio. In this species, irrespective of infection status, some mated females are constrained to produce extremely male-biased broods, whereas other females produce extremely female-biased broods. We analysed brood sex ratio of females mated with males of the same infection status at two temperatures. We found that at 20 °C the frequency of constrained sex allocation in coinfected pairs was reduced by 27% when compared to uninfected pairs. However, at 25 °C the constrained sex allocation frequency increased and became similar between coinfected and uninfected pairs, resulting in more male-biased population sex ratios at the higher temperature. This temperature-dependent pattern occurred without changes in endosymbiont densities and compatibility. Our findings indicate that endosymbionts affect sex ratios of haplodiploid hosts beyond the commonly recognised reproductive manipulations by causing female-biased sex allocation in a temperature-dependent fashion. This may contribute to a higher transmission efficiency of CI-inducing endosymbionts and is consistent with previous models that predict that CI by itself is less efficient in driving endosymbiont invasions in haplodiploid hosts.
Highlights
Inherited bacterial endosymbionts are common in nature and play pivotal roles in their hosts’ biology, ecology and evolution (Buchner 1965)
We examined the effect of bacterial endosymbionts on the sex allocation in Pezothrips kellyanus (Thripidae), a native Australian haplodiploid thrips species which is naturally infected with one Cardinium strain and one Wolbachia supergroup B strain (Nguyen et al 2016)
In summary, our study demonstrated that the role of common endosymbionts such as Wolbachia and Cardinium in host reproduction is beyond the induction of commonly recognized reproductive manipulations
Summary
Inherited bacterial endosymbionts are common in nature and play pivotal roles in their hosts’ biology, ecology and evolution (Buchner 1965). Wolbachia and Cardinium bacteria are ubiquitous endosymbionts of arthropods (Weinert et al 2015) and are inherited via the egg cytoplasm, while they are removed from sperm cells during spermatogenesis and, their paternal inheritance is very rare (Bressac and Rousset 1993; Doremus et al 2020). Their maternal transmission has resulted in the evolution of host reproductive manipulations that select for an increased proportion of infected females in host populations (Hurst and Frost 2015; Zug and Hammerstein 2015). While affecting reproductive systems of hosts, reproductive manipulators that manipulate sex ratios intrinsically affect sex allocation (Werren and Beukeboom 1998; Vala et al 2003), and manifestation of this can vary depending on the hosts’ sex determination system (Kageyama et al 2012)
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