Deep mtDNA divergences indicate cryptic species in a fig-pollinating wasp

Abstract

Background: Figs and fig-pollinating wasps are obligate mutualists that have coevolved for ca. 90 million years. They have radiated together, but do not show strict cospeciation. In particular, it is now clear that many fig species host two wasp species, so there is more wasp speciation than fig speciation. However, little is known about how fig wasps speciate.

Results: We studied variation in 71 fig-pollinating wasps from across the large geographic range of Ficus rubiginosa in Australia. All wasps sampled belong to one morphological species (Pleistodontes imperialis), but we found four deep mtDNA clades that differed from each other by 9-17% nucleotides. As these genetic distances exceed those normally found within species and overlap those (10-26%) found between morphologically distinct Pleistodontes species, they strongly suggest cryptic fig wasp species. mtDNA clade diversity declines from all four present in Northern Queensland to just one in Sydney, near the southern range limit. However, at most sites multiple clades coexist and can be found in the same tree or even the same fig fruit and there is no evidence for parallel sub-division of the host fig species. Both mtDNA data and sequences from two nuclear genes support the monophyly of the "P. imperialis complex" relative to other Pleistodontes species, suggesting that fig wasp divergence has occurred without any host plant shift. Wasps in clade 3 were infected by a single strain (W1) of Wolbachia bacteria, while those in other clades carried a double infection (W2+W3) of two other strains.

Conclusion: Our study indicates that cryptic fig-pollinating wasp species have developed on a single host plant species, without the involvement of host plant shifts, or parallel host plant divergence. Despite extensive evidence for coevolution between figs and fig wasps, wasp speciation may not always be linked strongly with fig speciation.

Figure 1

Distribution and frequencies of the four P. imperialis mtDNA clades. Distribution of clades 1 (white fill), 2 (hatch), 3 (gray) and 4 (black) in four regions across the range of F. rubiginosa. Sample sizes are given in parentheses above pie-charts. Numbers next to arrows refer to sampling sites listed in Table 1. Open and closed circles on the map reveal the distributions of F. rubiginosa f. rubiginosa and F. rubiginosa f. glabrescens, respectively. QLD = Queensland, NSW = New South Wales, VIC = Victoria, SA = South Australia.

Figure 2

Cytochrome b. Consensus Bayesian topology of a 444 bp region of the cytochrome b gene for 45 P. imperialis individuals, 14 other Pleistodontes species [33] and 4 outgroup Ceratosolen species [34]. Posterior node probabilities are indicated above each node. Thick branches represent >1 identical haplotypes.

Figure 3

28S rRNA. Neighbour-joining phylogram of a 1,060 bp region of the 28S rRNA gene for 23 P. imperialis individuals, 14 other Pleistodontes species and an outgroup taxon, Platyscapa soraria. Percentage bootstrap support (1000 replicates) is indicated above branches. Unlabelled branches had bootstrap support of less than 50%.