Female-biased sex ratios unrelated to Wolbachia infection in European species of the Jaera albifrons complex (marine isopods)

Abstract

Female-biased sex ratios and reproductive isolation in arthropods can be caused by endosymbiotic Wolbachia bacteria that manipulate the reproductive system of their host. Wolbachia is particularly common in terrestrial host species, but its frequency in marine arthropods is less well known. Here we asked whether Wolbachia bacteria are accountable for the female-biased sex ratio and variation in reproductive isolation levels observed in the four European species of the Jaera albifrons complex (marine intertidal isopods). We analysed the sex ratio in young adults reared in the laboratory (indicative of the sex ratio at birth), compared it with the adult sex ratio in natural populations, and performed a molecular survey of Wolbachia infection based upon amplification of three gene targets using 11 different protocols tested in 817 individuals from all species of the Jaera albifrons complex. One species (J. ischiosetosa) had a female-biased sex ratio at birth but showed no sign of infection by Wolbachia bacteria. This species, together with two others (J. albifrons and J. forsmani) also displayed female-biased sex ratio in adults in nature, while the adult sex ratio in the fourth European species (J. praehirsuta) was unbiased. A new Wolbachia strain was identified in J. albifrons and J. praehirsuta, albeit at very low frequency in populations. We conclude that Wolbachia bacteria are present in at least two species of the Jaera albifrons complex, but their prevalence is too low to have any effect on sex ratio and reproductive isolation. A sex ratio distorter other than Wolbachia may be acting in some J. ischiosetosa populations, and we hypothesize that the adult female excess seen in most species results from habitat-dependent, male-biased mortality in natural conditions.

Microbiome composition within a sympatric species complex of intertidal isopods (Jaera albifrons)

“…However, these results are fully compatible with previous studies that have failed to reliably detect Wolbachia infection in total Jaera DNA extracts using targeted PCR assays [ 48 , 49 ]. Ribardière et al [ 48 ] screened 817 individuals across the Jaera albifrons species complex using 11 PCR protocols, but found little evidence of infection beyond an ephemeral novel haplotype in some Jaera albifrons and Jaera praehirsuta individuals, identified using a nested PCR protocol. As such, infection of Jaera species by Wolbachia or other bacterial sex-ratio distorting parasites cannot be ruled out, but infection intensities and prevalence appear to be very low and difficult to detect.…”

“…Even if some of the identified sequences represented novel, somewhat diverged, strains of these reproductive parasites, all sequences in question had very low abundances and would not suggest high infection intensities. These results are difficult to reconcile with prevalent sex-ratio distortion in Jaera [ 46 – 48 ] and pervasive Wolbachia infection in many crustaceans [ 49 , 77 ]. However, these results are fully compatible with previous studies that have failed to reliably detect Wolbachia infection in total Jaera DNA extracts using targeted PCR assays [ 48 , 49 ].…”

“…These results are difficult to reconcile with prevalent sex-ratio distortion in Jaera [ 46 – 48 ] and pervasive Wolbachia infection in many crustaceans [ 49 , 77 ]. However, these results are fully compatible with previous studies that have failed to reliably detect Wolbachia infection in total Jaera DNA extracts using targeted PCR assays [ 48 , 49 ]. Ribardière et al [ 48 ] screened 817 individuals across the Jaera albifrons species complex using 11 PCR protocols, but found little evidence of infection beyond an ephemeral novel haplotype in some Jaera albifrons and Jaera praehirsuta individuals, identified using a nested PCR protocol.…”

“…The ingroup species are reproductively isolated through female preference for tactile courtship stimuli administered by males [ 41 , 42 ], genetic incompatibilities conferring rapid hybrid breakdown [ 41 , 42 ], and ecological zonation due to species-specific preferences of substrate, drainage, salinity and exposure [ 41 , 43 , 44 ], though some plasticity in the degree of reproductive isolation and frequency of introgressive hybridisation has been noted [ 41 , 45 ]. In spite of these reproductive isolation mechanisms, all species are polyphyletic according to the mitochondrial 16S rRNA gene and also usually display substantial sex-ratio bias towards females [ 46 – 48 ]. This would be consistent with the presence of sex-ratio distorting reproductive parasites and associated erosion of mitochondrial diversity [ 42 ].…”

“…This would be consistent with the presence of sex-ratio distorting reproductive parasites and associated erosion of mitochondrial diversity [ 42 ]. Attempts of detecting Wolbachia in Jaera via PCR have not yielded conclusive evidence for ongoing infection [ 48 , 49 ]. However, a proper characterisation of the Jaera microbiome via next-generation sequencing has not yet been attempted, thus reproductive parasites other than Wolbachia may be present and affect Jaera demography and evolution.…”