Megapode phylogeny and the interpretation of incubation strategies

Among megapodes, several incubation stategies can be recognized. Eggs are incubated by heat generated from microbial decomposition, volcanism, or the sun. For a long time, controversy has existed whether these strategies represent primitive traits, inherited from reptilian ancestors, or have evolved from the 'regular' way of incubation in birds. In this paper these strategies are interpreted by superimposing them on the most recent phylogenetic hypotheses regarding the inter- and intrafamilial relationships of megapodes. We conclude that similarities shared with reptiles and kiwis are due to convergence. Arguments are put forward that mound-building represents the plesiomorphic condition in megapodes and that burrow-nesting has been derived from it. Furthermore, we infer that burrow-nesting at sun-exposed beaches has evolved from burrow-nesting in volcanically heated soils. René W.R.J. Dekker, National Museum of Natural History, P.O. Box 9517, 2300 RA Leiden, The Netherlands

Cytoskeleton and gravity at work in the establishment of dorso-ventral polarity in the egg of Xenopus laevis

The establishment of polarities during early embryogenesis is essential for normal development. Amphibian eggs are appropriate models for studies on embryonic pattern formation. The animal-vegetal axis of the axially symmetrical amphibian egg originates during oogenesis and foreshadows the main body axis of the embryo. The dorso-ventral polarity is epigenetically established before first cleavage. Recent experiments strongly suggest that in the monospermic eggs of the anuran Xenopus laevis both the cytoskeleton and gravity act in the determination of the dorso-ventral polarity. In order to test the role of gravity in this process, eggs will be fertilized under microgravity conditions during the SL-D1 flight in 1985. In a fully automatic experiment container eggs will be kept under well-defined conditions and artificially fertilized as soon as microgravity is reached; eggs and embryos at different stages will then be fixed for later examination. Back on earth the material will be analysed and we will know whether fertilization under microgravity conditions is possible. If so, the relation of the dorso-ventral axis to the former sperm entry point will be determined on the whole embryos; in addition eggs and embryos will be analysed cytologically