Divergence of Drosophila Melanogaster Repeatomes in Response to a Sharp Microclimate Contrast in Evolution Canyon, Israel
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Divergence of Drosophila melanogaster repeatomes in response to a sharp microclimate contrast in Evolution Canyon, Israel Young Bun Kima, Jung Hun Ohb, Lauren J. McIvera, Eugenia Rashkovetskyc, Katarzyna Michalaka, Harold R. Garnera,d, Lin Kanga, Eviatar Nevoc,1,2, Abraham B. Korolc,1, and Pawel Michalaka,d,e,1,2 aVirginia Bioinformatics Institute and Departments of dBiological Sciences and eFish and Wildlife Conservation, Virginia Tech, Blacksburg, VA 24061; bDepartment of Medical Physics, Memorial Sloan–Kettering Cancer Center, New York, NY 10065; and cInstitute of Evolution, Haifa University, Haifa 3498838, Israel Contributed by Eviatar Nevo, June 4, 2014 (sent for review May 8, 2014; reviewed by Alan R. Templeton and Harmit S. Malik) Repeat sequences, especially mobile elements, make up large was polymorphic for a 1.2-kb P-element insertion, with the insert portions of most eukaryotic genomes and provide enormous, albeit being 28 times more frequent in NFS- than SFS-inhabiting flies. commonly underappreciated, evolutionary potential. We analyzed The P-element disruption was associated with decreased Hsp70 repeatomes of Drosophila melanogaster that have been diverging expression and heat-shock survival, but increased reproductive in response to a microclimate contrast in Evolution Canyon (Mount success in temperate conditions (13). Although elevated levels of Carmel, Israel), a natural evolutionary laboratory with two abutting Hsp70 are beneficial during thermotolerance challenges, they tend slopes at an average distance of only 200 m, which pose a constant to be deleterious for growth and development (14, 15). ecological challenge to their local biotas. Flies inhabiting the colder In parallel with a common pattern of slope-specific adapta- and more humid north-facing slope carried about 6% more trans- tions observed across many other taxa inhabiting this remarkable posable elements than those from the hot and dry south-facing ecological microgradient (16, 17), SFS-derived D. melanogaster slope, in parallel to a suite of other genetic and phenotypic differ- outperform their conspecific neighbors from NFS not only in ences between the two populations. Nearly 50% of all mobile ele- basal and inducible thermotolerance after diverse heat shocks ment insertions were slope unique, with many of them disrupting (18–20), but also in resistance to desiccation and starvation (18, coding sequences of genes critical for cognition, olfaction, and ther- 21). In addition to stress resistance, the two populations differ in motolerance, consistent with the observed patterns of thermotoler- oviposition site preferences (18), male courtship song parameters ance differences and assortative mating. (22), sexual and reproductive behavior (23) leading to partial assortative mating within slopes (24), and phenotypic plasticity for adaptive evolution | genome sequencing | microsatellite | wing morphology (25)—all this despite the physical proximity and incipient speciation migration between slopes (26). A recent genome analysis of the two populations (27) reveals ne of the greatest surprises of comparative genomics has a number of chromosomal regions of interslope divergence and Obeen the discovery that eukaryotic genomes are loaded with low sequence polymorphism, suggestive of selective sweeps among ubiquitous repeat sequences, such as transposable elements genes enriched for functions related to stimulus responses and (TEs) and tandem array repeats (satellites). Once relegated to developmental and reproductive processes, remarkably consistent junk DNA, today many repeat elements emerge as potent func- with our previous findings on the phenotypic patterns of stress tional genetic units, providing inspiration for new research ini- responses, life history, and mating functions. Sequence divergence tiatives (1). Although traditionally viewed as selfish DNA, TEs are increasingly being recognized as agents of adaptive change and a rich source of evolutionary novelties, including hybrid Significance dysgenesis (2), insecticide resistance (3), mammalian placenta (4), and vertebrate adaptive immune system (4), as well as flower Repeatome, or the ensemble of all repeat sequences, with its development and plant fitness (5). Domesticated TEs can be enormous variability and internal epigenetic dynamics, emerges repurposed to function as transcription factors, whereas others play as a critical source of potentially adaptive changes and evolu- various roles in heterochromatin formation, genome stability, cen- tionary novelties. This conclusion is exemplified here by the tromere binding, chromosome segregation, meiotic recombination, cosmopolitan Drosophila melanogaster from a sharp ecological TE silencing, programmed genome rearrangement, V(D)J recom- contrast in North Israel. Flies derived from the opposing sides bination, and translational regulation (4, 6). Even low-complexity of this long-studied microsite exhibit a significant difference in sequence repeats, such as microsatellites, may act as novel regula- the contents and distribution of mobile elements, as well as tory elements (7), enhance alternative splicing (8), and provide new microsatellite allele frequencies, corresponding well with ear- substrate for protein coding variation (9). lier reported phenotypic patterns of stress resistance and TEs have been implicated in adaptations to environmental assortative mating in the system. changes either through somewhat elusive direct remobilization Author contributions: E.N., A.B.K., and P.M. designed research; Y.B.K., E.R., and K.M. after stress (10) or more indirectly as a source of new and pre- performed research; Y.B.K., J.H.O., L.J.M., H.R.G., L.K., A.B.K., and P.M. analyzed data; existing genetic variation (11). A striking example of adaptive TE and Y.B.K., A.B.K., and P.M. wrote the paper. insertion polymorphism has been found in Drosophila mela- Reviewers: A.R.T., Washington University in St. Louis; and H.S.M., Fred Hutchinson Cancer nogaster from Evolution Canyon (lower Nahal Oren, Israel) (12, Research Center. 13), in which slopes 100–400 m apart differ dramatically in The authors declare no conflict of interest. aridity, solar radiation, and associated vegetation due to the 1E.N., A.B.K., and P.M. contributed equally to this work. higher insolation on the south-facing slope (SFS) relative to the 2To whom correspondence may be addressed. Email: [email protected] or pawel@ north-facing slope (NFS).The promoter region of heat-shock- vbi.vt.edu. protein-70Ba (hsp70Ba), one of the five Hsp70 paralogs that This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. encode a major inducible heat-shock protein in D. melanogaster, 1073/pnas.1410372111/-/DCSupplemental. 10630–10635 | PNAS | July 22, 2014 | vol. 111 | no. 29 www.pnas.org/cgi/doi/10.1073/pnas.1410372111 Downloaded by guest on September 29, 2021 Table 1. Chromosomal distribution of transposable elements in female germ-line mobilization in the progeny of certain D. mela- Evolution Canyon D. melanogaster nogaster intraspecific crosses, responsible for a maternal effect Chr. 5UTR 3UTR Intron CDS Promoter Intergenic Total embryonic lethality known as the I-R type of hybrid dysgenesis (32). Quasimodo and roo LTRs were the next most slope-di- SFS vergent TEs, followed by LINE-like jockey and Doc. The most X 26 51 778 78 20 1,054 2,007 slope-differential TIR was P-element, represented by 45 unique 2L 36 48 1,162 94 25 1,485 2,850 insertion sites. Interestingly, P-element insertions in the Hsp70Ba 2R 49 72 1,308 112 28 1,418 2,987 promoter reported earlier (12, 13) were absent in the current 3L 34 51 1,301 108 30 1,449 2,973 samples, a result of either demographic dynamics or selection 3R 44 80 1,262 120 30 1,261 2,797 against the insertion over the course of 13 y separating the fly 4 4 13 361 22 1 175 576 collections in the field. However, there were 76 other unique TE Total 193 315 6,172 534 134 6,842 14,190 insertions in SFS and 107 in NFS within the putative promoter NFS X 27 48 852 66 24 1,077 2,094 regions, including a P-element in a gene from the small heat shock 2L 32 46 1,276 93 33 1,642 3,122 protein (HSP20) family, Hsp67Ba, in SFS flies. Although expres- 2R 50 69 1,338 105 35 1,495 3,092 sion of Hsp67Ba was not directly investigated in D. melanogaster 3L 44 54 1,415 96 29 1,601 3,239 from Evolution Canyon, we have previously shown that small 3R 50 75 1,370 107 41 1,257 2,900 Hsps, such as Hsp40 and Hsp23, can contribute to thermotol- 4 0 12 369 21 2 174 578 erance in the system (33). There were at least 20 significant gene Total 203 304 6,620 488 164 7,246 15,025 ontology (GO) term enrichments representing genes with slope- unique TE insertions in their putative promoters, with functional Chr., chromosome; NFS, north-facing slope; SFS, south-facing slope. activities related to hydrolases and alternative splicing being most conspicuous (Tables S2 and S3). in genes underlying adaptive changes, coupled with assortative An additional 426 genes were disrupted by 511 slope-specific mating within populations, can be interpreted as signatures of TE insertions within coding sequences, presumably leading to ’ incipient speciation (21). The picture of adaptive divergence in the the genes functional inactivation. Interestingly, there were 20 natural system remains largely incomplete without looking into cognition-related and 17 sensory perception-related genes af- noncoding parts of the genomes, primarily those occupied by re- fected by the inserts, including 8 olfactory receptor and 8 gus- peat elements. To fill the void, we have now used high-coverage tatory receptor genes, all critical for detecting food and avoiding (40×) genome sequencing and analyzed variation of TEs and toxicants, as well as for courtship and mating. Cognition, sensory microsatellites in the two D. melanogaster populations. perception of chemical stimuli, and olfaction were among the most significantly overrepresented GO terms (Benjamini–Hochberg- Results and Discussion adjusted P < 0.01) among genes with TE-disrupted coding In D.