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

Area of Science:

• Evolutionary Biology
• Insect Ecology
• Speciation Research

Background:

• Figs and their pollinating wasps exhibit a 90-million-year-old obligate mutualism with extensive co-diversification.
• While figs and wasps have radiated together, wasp speciation often exceeds fig speciation, particularly when multiple wasp species inhabit a single fig species.
• The mechanisms driving fig wasp speciation remain largely unknown.

Purpose of the Study:

• To investigate the speciation processes in fig-pollinating wasps, specifically within the *Pleistodontes imperialis* complex.
• To determine if fig wasp speciation is linked to host plant shifts or independent divergence.
• To explore the genetic diversity and population structure of *Pleistodontes imperialis* across its geographic range.

Main Methods:

• Analysis of mitochondrial DNA (mtDNA) variation in 71 *Pleistodontes imperialis* wasps sampled across Australia.
• Sequencing of two nuclear genes to assess phylogenetic relationships.
• Investigation of Wolbachia bacterial infections within different wasp clades.

Main Results:

• Four deep mtDNA clades were identified within the morphologically defined *P. imperialis* species, with genetic distances suggesting cryptic speciation.
• mtDNA diversity decreased geographically, but multiple clades coexisted at most sites, including within the same fig tree or fruit.
• Genetic data supported the monophyly of the *P. imperialis* complex, indicating divergence occurred without host plant shifts. Different Wolbachia infection patterns were observed across clades.
• No evidence of parallel subdivision in the host fig species (*Ficus rubiginosa*) was found.

Conclusions:

• Cryptic fig wasp species have evolved on a single host plant species (*Ficus rubiginosa*) without host shifts or parallel host plant divergence.
• Wasp speciation can occur independently of fig speciation, challenging the notion of strictly linked co-evolutionary trajectories.
• This study highlights the complexity of speciation in obligate mutualisms and suggests that cryptic diversity is a significant factor in fig wasp evolution.