Figure 2

Characterization and expression of pea aphid torso-like genes in oviparous and viviparous development. A. Unrooted maximum-likelihood tree showing potential relationships between pea aphid (A. pisum) Tsl, Tslr1, and Tslr2 with several other arthropod torso-like homologs. Bootstrap percentages shown for all values >50%. B-I. tsl mRNA expression in oviparous (B-F) and viviparous (G-I) ovarioles. In oviparous ovarioles, tsl is expressed in a small group of somatic epithelial or follicle cells lying just posterior to late previtellogenic oocytes at the pedicel base (B). Arrowhead indicates domain in close-up (C). Observation of multiple focal planes showed that tsl is expressed in the follicle cells adjacent to the oocyte. The domain persists during vitellogenesis as oocyte becomes larger and cells undergo a series of morphological changes (D-F, arrowheads). The pedicel has been dissected away in D. tsl mRNA was not detected in any somatic epithelial or sheath cells surrounding viviparous oocytes (G, Stage 1). tsl is expressed in sheath cells only after embryo has begun to invaginate (Stage 8/9), first as a posterior circular domain at the base of the epithelial plug that is continuous with sheath cells surrounding the adjacent embryo (arrowheads in H, an optical cross section) and then as a non-circular domain following germ band invagination (I, arrowhead). In H,I the epithelial plug has been removed. J,K. tslr1 mRNA is found in viviparous oocytes (J), probably because it is expressed in viviparous trophocytes in the germarium before being deposited into the oocyte (K). K^′ shows nuclei stained with DAPI. ab, abdomen; cl, cephalic lobe; g, germarium; o, previtellogenic oocyte (nucleus marked with asterisk); ped, pedicel; sec/fc, somatic epithelial or follicle cells; sec/sc, somatic epithelial or sheath cells; th, thorax. Scale bars: 100 μm for B and D; 20 μm for C, E-K. Staging according to [8].