Aliee, M., Röper, J. C., Landsberg, K. P., Pentzold, C., Widmann, T. J., Jülicher, F., and Dahmann, C. Physical mechanisms shaping the Drosophila dorsoventral compartment boundary. (dresden-ipp.de)
psq mutations include lethal alleles (0482; Horowitz and Berg 1996), semilethal alleles that cause rudimentary ovaries and a defect in R3 / R4 photoreceptor specification (F112, E34, E39, D91; Weber et al. 1995), and viable female - sterile alleles causing defects in abdominal and dorsoventral patterning and in pole cell formation (fs1, I - 30, HK38, 2403, 2905, 6372, P1, P4, and X1 - 30; Schupbach and Wieschaus 1991; Siegel et al. 1993; Horowitz and Berg 1996). (genetics.org)
Regardless of its nature, dorsoventral axis inversion (35) would have brought the ventral midline tissue into a dorsal position in the chordate lineage, and the appearance of incompressible vacuoles (14) would have gradually transformed it into a stiff rod of constant length; the amphioxus notochord could then be regarded a vestige of a contractile - cartilaginous transition. (scienceintheclassroom.org)