Evo Devo Projects
Different strategies are used in insects to establish embryonic polarity. In the ancestral short-germ mode of development, nuclei fated to become the embryo are restricted to the posterior end of the egg while the anterior of the egg develops as extra-embryonic membranes. Only anterior segments are patterned at the syncytial blastoderm while abdominal segments form in a posterior growth zone. This system relies on a single posterior morphogenetic center whereby a localized posterior determinant (nanos) is responsible for forming gradients of factors that pattern head and thorax. Posterior segments form through a vertebrate-like segmentation clock. In the derived long-germ mode, the embryo occupies the entire egg and all segments form simultaneously in a syncytial environment. In Drosophila, this is made possible by a gradient of Bicoid that emanates from the anterior of the egg. However, Bicoid is a new invention of higher Diptera and is not found in any other phyla. We analyzed axis formation in Nasonia, a wasp (Hymenoptera) that has developed long germ independently from Drosophila. Like bicoid in Drosophila, Nasonia orthodenticle (Nv-otd1) maternal mRNA is localized at the anterior of the egg where it generates a morphogenetic gradient. Maternal Nv-otd1 mRNA is also localized at the posterior of the egg and is required for its patterning. Like Bicoid, Nv-Otd differentially activates anterior genes. It also induces a change in developmental timing by activating early expression of posterior genes whose expression is delayed to the growth zone in short germ insects. [Lynch Nature 2006; Olesnicky Development 2006; Brent Science 2007; Lynch Development 2010; Werren Science 2010].
Pair rule genes in Nasonia: (Miriam Rosenberg) Signaling from maternal and gap genes are integrated at the level of the pair rule genes in Drosophila whose promoters carry both stripe specific enhancer elements that translate local TF concentration of upstream genes into segment identity and position along the A-P axis. We are working to understand how known gap and maternal genes regulate the Nasonia pair rule genes. Nasonia even-skipped exhibits characteristics of both long- and short-germ insects in its segmental expression and function. We are currently establishing how this pattern is achieved by upstream genes and aim to identify enhancers that control eve expression. We are also studying how other pair rule genes are expressed and regulated in Nasonia. Our loss of function data for this class of genes support a model in which there are two modes of segment patterning in Nasonia: an anterior, simultaneous mode that resembles that of Drosophila, and a posterior sequential mode that resembles that of ancestral (short germ) insects. It will be of paramount interest to investigate how these two modes can co-exist and resolve during the evolution of long germ embryogenesis.
The ant project: (Giacomo Mancini) The complexity of social organization in ants is in stark contrast with their relatively simple nervous system. How can a small brain drive distinct caste behaviors, identify complex chemical cues that encode colony identity, and social status of each individual? One feature that makes the ant Harpegnathos saltator amenable to a model organism is the presence of gamergates, or pseudoqueens. Once a queen is removed from the colony, workers transform into gamergates whose sole purpose is to lay eggs. Gamergates also undergo a drastic reduction in brain size that is especially marked for the optic lobes. We are investigating the neurodegenerative process observed in the optic lobes of H. saltator gamergates and testing whether it is due to a primary effect on the retina.
Furthermore, as a modern model organism cannot succeed without the ability to manipulate genes with mutant and transgenic lines that can be propagated in the laboratory. We use CRISPR, TALENs and zinc-finger nucleases (ZFNs) to generate mutants by disrupting candidate genes for optic lobe degeneration and behavior. For reliable transgenesis, we are generating "acceptor" H. saltator lines by inserting ΦC31(AttP) landing sites in the genome using TALEN and CRISPR knock-in. This will allow us to introduce any desired sequence by recombination with DNA containing AttB donor sites. Combining this with tissue specific expression system (for example GAL4/UAS) will allow us to target overexpression, rescue, or knockdown to particular tissues of interest.