Homeosis

The first Atelocerata were machilid-like insects descended from Palaeocar-idacea (Malacostraca Syncarida) via neoteny, homeosis and gamoheterotopy. In these littoral hoppers, the five posterior segments of the malacostracan thorax were repatterned after abdominal ones to maintain the caridoid escape reaction at uncompensated gravity out of water. Their modified gonopods were shifted caudally and transferred from males to females to form an ovipositor. The vigorously flexing abdomen turned useless in cryptic habitats and was homeotically repatterned after the thorax in myriapods, which evolved as litter creepers from bristletail ancestors via entognathous hexapods.

The inflorescence of Philodendron constitutes an interesting morphological model to analyse the phenomenon of homeosis quantitatively at the floral level. The specific goals of this study were (1) to characterize and quantify the range of homeotic transformation in Philodendron billietiae, and (2) to test the hypothesis that the nature of flowers surrounding atypical bisexual flowers (ABFs) channel the morphological potentialities of atypical bisexual flowers. Inflorescences of P. billietiae at different stages of development were observed using SEM. The number of appendices in male, female and sterile flowers were counted on 11 young inflorescences (5-6 flowers per inflorescence). The number of staminodes and carpels on ABFs were counted on 19 inflorescences (n = 143). These data were used for regression and ANOVA analyses. There was an average of 4.1 stamens per male flower, 9.8 carpels per female flower and 6.8 staminodes per sterile male flower. There was an average of 7.3 flora...

Summary: In modern vertebrates upper and lower jaws are morphologically different. Both develop from the mandibular arch, which is colonized mostly by Hox-free neural crest cells. Here we show that simultaneous inactivation of the murine homeobox genes Dlx5 and Dlx6 results in the transformation of the lower jaw into an upper jaw and in symmetry of the snout. This is the first homeotic-like transformation found in this Hox-free region after gene inactivation. A suggestive parallel comes from the paleontological record, which shows that in primitive vertebrates both jaws are essentially mirror images of each other. Our finding supports the notion that Dlx genes are homeotic genes associated with morphological novelty in the vertebrate lineage. genesis 34:221–227, 2002. © 2002 Wiley-Liss, Inc.