Multiple hybridization events and repeated evolution of homoeologue
expression bias in parthenogenetic, polyploid New Zealand stick insects
Abstract
During hybrid speciation, homoeologues combine in a single genome.
Homoeologue expression bias (HEB) occurs when one homoeologue has higher
gene expression than another. HEB has been well characterized in plants
but rarely investigated in animals, especially invertebrates.
Consequently, we have little idea as to the role that HEB plays in
allopolyploid invertebrate genomes. If HEB is constrained by features of
the parental genomes, then we predict repeated evolution of similar HEB
patterns among hybrid genomes formed from the same parental lineages. To
address this, we reconstructed the history of hybridization between the
New Zealand stick insect genera Acanthoxyla and Clitarchus using a
high-quality genome assembly from Clitarchus hookeri to call variants
and phase alleles. These analyses revealed the formation of three
independent diploid and triploid hybrid lineages between these genera.
RNA sequencing revealed a similar magnitude and direction of HEB among
these hybrid lineages, and we observed that many enriched functions and
pathways were also shared among lineages, consistent with repeated
evolution due to parental genome constraints. In most lineages, a slight
majority of the genes involved in mitochondrial function showed HEB
towards the maternal homoeologues, consistent with only weak effects of
mitonuclear incompatibility. We also observed a proteasome functional
enrichment in most lineages and hypothesize this may result from the
need to maintain proteostasis in hybrid genomes. Reference bias was a
pervasive problem, and we caution against relying on HEB estimates from
a single parental reference genome.