Intraspecific variation in gene expression under prolonged drought in Piriqueta hybrids and their parental taxa

Closely related lineages can possess phenotypic variation important for adaptation and the evolution of new species-a phenomenon that can be more clearly elucidated by studying hybrid generations. We compare variation in gene expression in response to drought for two taxa and advanced-generation hybrids of the Piriqueta cistoides ssp. caroliniana complex that differ in their levels of tolerance to water limitation. Drought treatments lasted 36 days, through four cycles of drought. Gene expression in drought and control treatments was assessed using heterologous hybridization to a Glycine max microarray. There was a predominance of down-regulated genes in response to sustained drought in all morphotypes. This pattern was more pronounced in the hybrids, which can exhibit greater drought tolerance under field and greenhouse conditions than the parental morphotypes. Expression response profiles were more similar between the hybrid and the drought-tolerant parental morphotype than they were between the hybrid and the parental morphotype that occurs in more mesic habitats. Predominant down-regulation of gene expression contrasts with studies of response to short-term drought and with studies of drought response in annual and non-drought-tolerant model species, and supports the results of a growing number of studies with other drought-tolerant perennial plants under prolonged drought.