The interplay between prior selection, mild intermittent exposure, and
acute severe exposure in phenotypic and transcriptional response to
hypoxia
Abstract
Hypoxia has profound and diverse effects on aerobic organisms,
disrupting oxidative phosphorylation and activating several protective
pathways. Predictions have been made that exposure to mild intermittent
hypoxia may be protective against more severe exposure and may extend
lifespan. Both effects are likely to depend on prior selection on
phenotypic and transcriptional plasticity in response to hypoxia, and
may therefore show signs of local adaptation. Here we report the
lifespan effects of chronic, mild, intermittent hypoxia (CMIH) and
short-term survival in acute severe hypoxia (ASH) in four clones of
Daphnia magna originating from either permanent or intermittent
habitats, the latter regularly drying up with frequent hypoxic
conditions. We show that CMIH extended the lifespan in the two clones
originating from intermittent habitats but had the opposite effect in
the two clones from permanent habitats, which also showed lower
tolerance to ASH. Exposure to CMIH did not protect against ASH; to the
contrary, Daphnia from the CMIH treatment had lower ASH tolerance than
normoxic controls. Few transcripts changed their abundance in response
to the CMIH treatment in any of the clones. After 12 hours of ASH
treatment, the transcriptional response was more pronounced, with
numerous protein-coding genes with functionality in mitochondrial and
respiratory metabolism, oxygen transport, and, unexpectedly,
gluconeogenesis showing up-regulation. While clones from intermittent
habitats showed somewhat stronger differential expression in response to
ASH than those from permanent habitats, there were no significant
hypoxia-by-habitat of origin or CMIH-by-ASH interactions. GO enrichment
analysis revealed a possible hypoxia tolerance role by accelerating the
molting cycle and regulating neuron survival through up-regulation of
cuticular proteins and neurotrophins, respectively.