Molecular and epigenetic regulation of seasonal reproduction in Terai tree frog (Polypedates teraiensis)

Abstract

Seasonal breeders predominantly use photoperiod as the predictable environmental cue to time their reproduction. Terai tree frogs are long-day seasonal breeders, but the molecular mechanism is unknown. We tested the role of different photoperiodic conditions on expression levels of candidate genes involved in seasonal reproduction and epigenetic regulation. Four experiments were performed. In experiment 1, frogs were exposed to long (LD: 16L:8D) or short photoperiod (SD: 8L:16D). In experiment 2, animals were procured at four different phases of breeding, i.e., during April (emergence just after hibernation), June (breeding phase), August (post-breeding), and October (just before hibernation). In experiments 3 and 4, frogs were exposed to equinox photoperiod but different (10, 100, or 500 lx) light intensities (exp. 3) or wavelength (red: 640 nm, green: 540 nm, blue: 450 nm or white; exp. 4). After 2 weeks, animals were euthanized, and their brain was harvested. mRNA levels of transcripts involved in photoperiodic transduction (Eya3 and Opn5), reproduction (Tshß, GnRH, Dio2, and Dio3), and epigenetics regulation (Dnmt1, Dnmt3a, Hdac1, Hdac3, and Tet2) were measured. Results show that LD promotes the upregulation of Eya3, Opn5, Tshß, GnRH, and Dio2. Differential expression of Opn5 during LD and SD suggests its involvement in light perception. Dio3 levels were upregulated in SD (exp.1) and during the post-breeding phase (exp. 2). These results employ the limited role of light intensity and spectrum in reproduction. This is the first study showing molecular machinery involved in the amphibian system's seasonal reproduction and epigenetic regulation.

Light receptors in the avian brain and seasonal reproduction

“…Concurrent with the increase in DIO2 expression, there is a decrease in type 3 deiodinase (DIO3) expression. This reciprocal switch in hypothalamic deiodinase expression between long and short photoperiods has been confirmed in a number of seasonal breeders (Borah et al, 2022;Nakao et al, 2008;Pérez et al, 2021;Stevenson & Ball, 2012;Wang et al, 2019;Yasuo et al, 2005). Deiodinase enzymes are key regulators of thyroid hormones with DIO3 acting to inactivate both bioactive triiodothyronine (T3) and thyroxine (T4), while DIO2 serves as an activating enzyme converting T4, abundant in circulation but low bioactivity, into bioactive T3.…”

Epigenetic Regulation of miR-25 and Lnc107153 on Expression of Seasonal Estrus Key Gene CHGA in Sheep

“…Recent studies have revealed the role of epigenetics in regulating the expression of seasonal reproductive key genes [10][11][12][13][14][15]. For example, it was reported that a short photoperiod inhibited hypothalamic DNA methyltransferase expression and reduced dio3 promoter DNA methylation, which up-regulated dio3 expression and induced gonadal regression [13].…”