Elevated Temperatures Cause Transposon-Associated DNA Damage in C. elegans Spermatocytes

Sexually reproducing organisms use meiosis to generate haploid gametes and faithfully transmit their genome to the next generation. In comparison to oogenesis in many organisms, spermatogenesis is particularly sensitive to small temperature fluctuations, and spermatocytes must develop within a very narrow isotherm [1Houston B.J. Nixon B. Martin J.H. De Iuliis G.N. Trigg N.A. Bromfield E.G. McEwan K.E. Aitken R.J. Heat exposure induces oxidative stress and DNA damage in the male germ line.Biol. Reprod. 2018; 98: 593-606Crossref PubMed Scopus (66) Google Scholar, 2Kim B. Park K. Rhee K. Heat stress response of male germ cells.Cell. Mol. Life Sci. 2013; 70: 2623-2636Crossref PubMed Scopus (83) Google Scholar, 3Sage T.L. Bagha S. Lundsgaard-Nielsen V. Branch H.A. Sultmanis S. Sage R.F. The effect of high temperature stress on male and female reproduction in plants.Field Crops Res. 2015; 182: 30-42Crossref Scopus (87) Google Scholar, 4Rohmer C. David J.R. Moreteau B. Joly D. Heat induced male sterility in Drosophila melanogaster: adaptive genetic variations among geographic populations and role of the Y chromosome.J. Exp. Biol. 2004; 207: 2735-2743Crossref PubMed Scopus (109) Google Scholar]. Although failure to thermoregulate spermatogenetic tissue and prolonged exposure to elevated temperatures are linked to male infertility in several organisms, the mechanisms of temperature-induced male infertility have not been fully elucidated [5Durairajanayagam D. Agarwal A. Ong C. Causes, effects and molecular mechanisms of testicular heat stress.Reprod. Biomed. Online. 2015; 30: 14-27Abstract Full Text Full Text PDF PubMed Scopus (201) Google Scholar]. Here, we show that upon exposure to a brief 2°C temperature increase, Caenorhabditis elegans spermatocytes exhibit up to a 25-fold increase in double-strand DNA breaks (DSBs) throughout meiotic prophase I and a concurrent reduction in male fertility. We demonstrate that these heat-induced DSBs in spermatocytes are independent of the endonuclease SPO-11. Further, we find that the production of these heat-induced DSBs in spermatocytes correlate with heat-induced mobilization of Tc1/mariner transposable elements, which are known to cause DSBs and alter genome integrity [6Hickman A.B. Dyda F. DNA transposition at work.Chem. Rev. 2016; 116: 12758-12784Crossref PubMed Scopus (39) Google Scholar, 7Wallis D.C. Nguyen D.A.H. Uebel C.J. Phillips C.M. Visualization and quantification of transposon activity in Caenorhabditis elegans RNAi pathway mutants.G3 (Bethesda). 2019; 9: 3825-3832Crossref PubMed Scopus (9) Google Scholar]. Moreover, we define the specific sequences and regions of the male genome that preferentially experience these heat-induced de novo Tc1 insertions. In contrast, oocytes do not exhibit changes in DSB formation or Tc1 transposon mobility upon temperature increases. Taken together, our data suggest spermatocytes are less tolerant of higher temperatures because of an inability to effectively repress the movement of specific mobile DNA elements that cause excessive DNA damage and genome alterations, which can impair fertility.