Changing sexes in a complex environment ‐ shifts in skeletal myosin isoforms in coastal populations of California sheephead fish

Abstract

Myosin heavy chain (MyHC) isoform expression was investigated in California sheephead, Semicossyphus pulcher, a protogynous hermaphroditic fish found in coastal Southern California and Northern Mexico. This species was collected from nine capture sites near the Channel Islands where both overall size, and size of individuals at the transitional sexual stage, varies considerably. We therefore cloned MyHC isoforms and analyzed proteins by SDS‐PAGE to determine if gender, size, water temperature, or geographic location influenced contractile protein expression. The relative percentage of the fast MyHC isoform decreased as sheephead fish transitioned from female to male. The highest expression of slow MyHC isoform was observed in male sheephead. The lowest expression of slow MyHC was seen in Northern and off‐shore female populations, when compared to Southern and near‐shore female populations. Relative percentage of slow MyHC isoform positively correlated with standard length. Thus, in this interesting system where muscle mass undergoes dramatic growth accompanying gender transitions, MyHC isoforms are likely influenced by both sexual development and environment.