ArHsp40 and ArHsp40-2 contribute to stress tolerance and longevity in Artemia franciscana , but only ArHsp40 influences diapause entry.

Embryos of the crustacean, Artemia franciscana , develop either ovoviviparously or oviparously, respectively yielding swimming larvae (nauplii) or encysted gastrulae (cysts). Nauplii molt several times and become adults whereas cysts enter diapause, a state of dormancy characterized by exceptionally low metabolism and high stress tolerance. Synthesis of molecular chaperones such as the J-domain proteins, ArHsp40 and ArHsp40-2 occurs during embryo development and post-diapause growth of A. franciscana and they influence development and stress tolerance. To further investigate J-domain protein function ArHsp40 and ArHsp40-2 were each knocked down by RNA interference. Reductions in ArHsp40 and ArHsp40-2 had no effect on adult survival, time to release of cysts and nauplii from females and first brood size. However, knockdown of both A. franciscana J-domain proteins reduced the longevity and heat tolerance of nauplii with the loss of ArHsp40 having a greater effect. The knockdown of ArHsp40, but not of ArHsp40-2, caused approximately 50% of cysts to abort diapause entry and hatch without exposure to an exogenous signal such as low temperature and/or desiccation. Cysts lacking ArHsp40 that entered diapause exhibited decreased stress tolerance as did cysts with reduced ArHsp40-2, the latter to a lesser degree. The longevity of nauplii hatching prematurely from cysts was less than for nauplii arising by other means. The results expand our understanding of Hsp40 function during A. franciscana stress tolerance and development, especially during diapause, and they provide the first example of a molecular chaperone that influences diapause entry.