Melanism in the spider Pityohyphantes phrygianus (C. L Koch): the genetics and the occurrence of different colour phenotypes in a natural population

Abstract

In the sheetweb spider Pityohyphantes phrygianus (C. L. Koch) there is a continuous variation in dark colouration. The specimens may be divided into three main phenotype classes of colouration; pale, intermediate and melanic. Analyses of the genetic basis of colour variation in P. phrygianus showed that a polygenic system is probably involved. The narrow heritability of dark colouration was estimated as 0·43 (±0·25), based on parent-offspring regression using mid-parent and mean offspring scores. A small-scale laboratory experiment indicated that the melanics may have an activity—and hence a foraging—advantage over non-melanics at low temperatures. A temporal variation in the frequencies of the three main phenotype classes was observed in a natural population in SW Sweden. This variation was due to differences between the proportion of pale and intermediate specimens in three cohorts; the proportion of melanics was low in all samples, averaging 3·7 per cent.