Female silkworms have the sex factor

Abstract

Exceptions to the WZ system include 'primitive' moths such as the Micropterigidae and Hepialidae (which arose before the evolution of W and so lack this chromosome and have a Z/ZZ system like their closest relatives, caddisflies) and some 'advanced' species, such as the Eri silkworm Samia cynthia ricini, in which the W chromosome has been lost8. Because W chromosomes are transmitted only through females, they cannot undergo recombination - a mechanism that generates genetic variation in other chromosome pairs, and that also prevents mutations that negatively affect gene function from becoming fixed in the population.