Variation in gene expression across infection status and elevation in a Hawaiian honeycreeper

Introduced pathogens exert novel selection on hosts, and although many host species have experienced drastic population declines in the absence of adaptation, some hosts have adapted to highly virulent pathogens. For instance, mosquitoes and Plasmodium relictum introduced to the Hawaiian Islands have resulted in extinctions and catastrophic population declines due to avian malaria, particularly in the diverse clade of Hawaiian honeycreepers. However, some species, such as the Hawai'i 'amakihi (Chlorodrepanis virens), can survive infection. Immunity exists in low-elevation populations where mosquitoes are abundant, whereas high-elevation, unexposed populations of 'amakihi display greatly reduced immunity. To explore the basis of adaptation to P. relictum in low-elevation 'amakihi, we sequenced transcriptomes from 24 low-elevation and 15 high-elevation 'amakihi. We tested for differential gene expression between (i) infected and uninfected birds and (ii) low- and high-elevation birds. Infected birds showed significant differences in expression across many transcripts with diverse cellular functions involved in different pathways of immune response; eight of the top 13 transcripts blasted to genes previously implicated in immunity to malaria in 'amakihi, and 11 have been identified in other infectious disease systems. Thirteen transcripts showed a trend of higher expression in high-elevation birds. These transcripts blasted to genes involved in metabolism, blood coagulation, and immune response. Our results provide increasing support for a subset of genes involved in immunity to malaria in 'amakihi and hint at possible antagonistic interactions between response to pathogens and environmental characteristics associated with elevation. Further work clarifying the nature of these interactions could benefit conservation efforts of Hawaiian honeycreepers in upper elevation refugia that are increasingly subject to malaria exposure.