Sciara

Meiosis I in Sciara males offers the only naturally occurring example of a monopolar spindle (panel A), allowing exploration of the broader question “How do chromosomes move on spindles?” Instead of random segregation of homologous chromosomes, all the paternal chromosomes move “backwards” away from the single pole and are discarded in a bud (panel B), raising the question of “How does the cell imprint the paternal and maternal origin of chromosome?” The germ-line limited L chromosomes found in Sciara all move to the single pole, posing the question: “How do the L chromosomes escape imprinting?”

An asymmetric bipolar spindle with a single centrosome forms in meiosis II of Sciara males (panel C) and all chromosomes move to the metaphase plate except for the X dyad that remains near the single centrosome. What governs non-disjunction of the X? This is regulated by a genetic locus called the “controlling element”, but how does it work?

The nullo-X product of meiosis II is discarded in a bud (panel D). Fertilization of the haploid egg by a double X sperm results in a triple X zygote. What is the mechanism for chromosome elimination in early embryos of one (in females) or two (in males) X chromosomes that are paternally imprinted and marked by the controlling element? How does sex determination work in Sciara that lacks a Y chromosome and mothers determine the sex of their offspring?

DNA puffs in Sciara salivary gland polytene chromosomes are loci of developmentally regulated site-specific DNA amplification. What is the mechanism that overrides the normal cellular controls that origins of DNA replication are activated no more than once per S phase?