Developmental profile and hormonal regulation of the transcription factors broad and Krüppel homolog 1 in hemimetabolous thrips

Abstract

In holometabolous insects, Krüppel homolog 1 (Kr-h1) and broad (br) are key players in the juvenile hormone (JH) regulation of metamorphosis: Kr-h1 is an early JH-response gene, while br is a transcription factor that directs pupal development. Thrips (Thysanoptera) are classified as hemimetabolous insects that develop directly from nymph to adult, but they have quiescent and non-feeding stages called propupa and pupa. We analyzed the developmental profiles of br and Kr-h1 in the western flower thrips Frankliniella occidentalis (Thripidae) that has one propupal instar and one pupal instar, and Haplothrips brevitubus (Phlaeothripidae) that has one propupal instar and two pupal instars, i.e. pupa I and pupa II. In F. occidentalis, the br mRNA levels were moderate in the embryonic stage, high at the larva-propupa transition, and low in the pre-final larval instar and the pupal stage, while Kr-h1 mRNA levels were high in the embryonic stage, remained at a moderate level in the larval and propupal stages, and low in the pupal stage. The expression profiles in H. brevitubus were very similar to those in F. occidentalis, except that the increase of br expression in the final larval stage occurs more slowly in H. brevitubus, and that the mRNA levels of br and Kr-h1 remained high in pupa I of H. brevitubus and then decreased. These profiles of br and Kr-h1 were comparable to those in holometabolous insects, although br expression found in thrips' embryogenesis is reminiscent of several hemimetabolous species. Treatment with an exogenous JH mimic (JHM) in distinct developmental stages consistently resulted in lethality as pupa of F. occidentalis or pupa II of H. brevitubus. Treatment with JHM to newly molted propupae caused prolonged expression of Kr-h1 and br in both species, suggesting that Kr-h1 and br could be involved in mediating anti-metamorphic signals of JHM.