Pervasive heteroplasmy in an invasive ambrosia beetle (Scolytinae) in southern California

Heteroplasmy, the presence of multiple distinct mitochondrial genotypes
(mitotypes) within an individual, has long been thought to be a rare
aberrance that is quickly removed by selection or drift. However,
heteroplasmy is being reported in natural populations of eukaryotes with
increasing frequency, in part due to the development of better diagnostic
molecular methods. Here, we report a seemingly stable heteroplasmic state
in California populations of the polyphagous shothole borer (PSHB),
Euwallacea fornicatus; an exotic invasive ambrosia beetle that is causing
significant tree dieback. We develop and validate a qPCR assay that
utilizes novel locked nucleic acid probes to detect different mitotypes,
and qualitatively assess heteroplasmy in individual PSHB. We prove the
utility of this assay by: a) mitotyping field-collected PSHB to document
the prevalence of heteroplasmy across its invasive range in southern
California; and, b) measuring relative titers of eac h mitotype across
multiple generations of heteroplasmic PSHB colonies in the laboratory to
assess the stability of transmission through the maternal germline. We
rule out the possibility that heteroplasmy is due to a NUMT, and find that
heteroplasmic individuals are common in Californian field populations of
PSHB, and that heteroplasmy persists in a stable state for at least 10
generations in experimental colonies. We also looked for evidence of the
common occurrence of paternal leakage, but found none. In light of our
results, we discuss competing hypotheses as to how heteroplasmy may have
first arisen, and continues to perpetuate, in Californian PSHB
populations.