Nutritional Stress Modulates Developmental Thresholds in a Male Dimorphic Mite

ABSTRACT Global warming can affect arthropods not only through direct thermal stress but also by altering nutritional conditions that shape development. For condition‐dependent alternative phenotypes, such environmental effects may change phenotype expression by shifting the distribution of individual condition, or by altering the developmental threshold at which individuals switch between phenotypes. I tested these alternatives in the bulb mite Rhizoglyphus robini , in which males develop into either fighters or scramblers depending on body size at a critical juvenile stage. Using isofemale lines reared on high‐quality yeast or low‐quality oats, I quantified male morph expression across body sizes and estimated diet‐specific switch points. Fighter expression was strongly condition‐dependent, with larger juveniles more likely to develop into fighters. However, the nutritional environment affected morph expression even after accounting for body size: males reared on oats were more likely to become fighters than same‐sized males reared on yeast. The size‐dependent relationship between body size and morph expression did not differ between diets, but the threshold for fighter expression shifted, with higher thresholds in yeast than in oats. The magnitude of this threshold shift also varied among isofemale lines. These results show that nutritional conditions influence morph expression not only by changing body size, but also by modifying the developmental threshold and its responsiveness across genetic backgrounds. Developmental thresholds may therefore represent environmentally responsive decision points through which arthropods buffer stress and generate variation relevant to adaptation under climate change.