Basal hsp70 expression levels do not explain adaptive variation of the warm- and cold-climate O3 + 4 + 7 and OST gene arrangements of Drosophila subobscura.

BACKGROUND: Drosophila subobscura exhibits a rich inversion polymorphism, with some adaptive inversions showing repeatable spatiotemporal patterns in frequencies related to temperature. Previous studies reported increased basal HSP70 protein levels in homokaryotypic strains for a warm-climate arrangement compared to a cold-climate one. These findings do not match the similar hsp70 genomic organization between arrangements, where gene expression levels are expected to be similar. In order to test this hypothesis and understand the molecular basis for hsp70 expression, we compared basal hsp70 mRNA levels in males and females, and analysed the 5' and 3' regulatory regions of hsp70 genes in warm- and cold-climate isochromosomal O3 + 4 + 7 and OST lines of D. subobscura. RESULTS: We observed comparable mRNA levels between the two arrangements and a sex-biased hsp70 gene expression. The number of heat-shock elements (HSEs) and GAGA sites on the promoters were identical amongst the OST and O3 + 4 + 7 lines analysed. This is also true for 3' AU-rich elements where most A and B copies of hsp70 have, respectively, two and one element in both arrangements. Beyond the regulatory elements, the only notable difference between both arrangements is the presence in 3' UTR of a 14 bp additional fragment after the stop codon in the hsp70A copy in five O3 + 4 + 7 lines, which was not found in any of the six OST lines. CONCLUSIONS: The equivalent hsp70 mRNA amounts in OST and O3 + 4 + 7 arrangements provide the first evidence of a parallelism between gene expression and genetic organization in D. subobscura lines having these arrangements. This is reinforced by the lack of important differential features in the number and structure of regulatory elements between both arrangements, despite the genetic differentiation observed when the complete 5' and 3' regulatory regions were considered. Therefore, the basal levels of hsp70 mRNA cannot account, in principle, for the adaptive variation of the two arrangements studied. Consequently, further studies are necessary to understand the intricate molecular mechanisms of hsp70 gene regulation in D. subobscura.

Chromosomal inversions promote genomic islands of concerted evolution of Hsp70 genes in the Drosophila subobscura species subgroup.

Heat-shock (HS) assays to understand the connection between standing inversion variation and evolutionary response to climate change in Drosophila subobscura found that "warm-climate" inversion O3+4 exhibits non-HS levels of Hsp70 protein like those of "cold-climate" OST after HS induction. This was unexpected, as overexpression of Hsp70 can incur multiple fitness costs. To understand the genetic basis of this finding, we have determined the genomic sequence organization of the Hsp70 family in four different inversions, including OST , O3+4 , O3+4+8 and O3+4+16 , using as outgroups the remainder of the subobscura species subgroup, namely Drosophila madeirensis and Drosophila guanche. We found (i) in all the assayed lines, the Hsp70 family resides in cytological locus 94A and consists of only two genes, each with four HS elements (HSEs) and three GAGA sites on its promoter. Yet, in OST, the family is comparatively more compact; (ii) the two Hsp70 copies evolve in concert through gene conversion, except in D. guanche; (iii) within D. subobscura, the rate of concerted evolution is strongly structured by inversion, being higher in OST than in O3+4 ; and (iv) in D. guanche, the two copies accumulated multiple differences, including a newly evolved "gap-type" HSE2. The absence of concerted evolution in this species may be related to a long-gone-unnoticed observation that it lacks Hsp70 HS response, perhaps because it has evolved within a narrow thermal range in an oceanic island. Our results point to a previously unrealized link between inversions and concerted evolution, with potentially major implications for understanding genome evolution.