Maternal effects on encystment in crosses between two geographic strains of Artemia franciscana

Maternal effects can have environmental or genetic causes. A method that can be used to demonstrate the genetic basis of a maternal effect is to look for grandfather effects in a backcross following reciprocal crosses. The absence of a grandfather effect would exclude a chromosomal basis for the maternal effect when the male sex is heterogametic (XX-XY sex determination system). However, in organisms in which the female is heterogametic (ZW-ZZ sex determination system), the absence of a grandfather effect does not rule out a chromosomal basis of the maternal effect, since the genes responsible for that effect can be located in the W chromosome, which is transmitted matrilineally. Conversely, the absence of a grandfather effect would point to a W-chromosome basis for the trait, provided that a maternal effect has been previously demonstrated. Distinguishing between W-located and autosome or Z-located maternal effects is important to understand the evolutionary dynamics of a trait. Here we report on a study of the chromosomal basis of maternal effects on two life-history traits related to encystment in the branchiopod crustacean Artemia franciscana, in which females are heterogametic. We performed crosses of two populations that differ in the number of cysts they produce. The proportion of encysted broods showed a maternal effect and was not affected by the grandfather's genotype, pointing to a W-chromosome basis. The average number of encysted offspring per brood showed a strong paternal effect and also a slight maternal effect. This trait also showed a grandfather effect, which suggests that the geographical variation has an autosomal or Z-chromosomal basis.