Untersuchungen zu Ampassung und Artbildung bei Pycnotropis tida (Diplopoda: Polydesmida: Aphelidesmidae: Amplininae) in amazonischen Überschwemmungswäldern / Katrin Vohland.

Tese (doutor) - Christian-Albrechts-Universität, 1998

The hypothesis of this work was that the flood pulse, which regularly causes inundations of wide forest areas along the Amazon, leads to genetic differentiation within species or even to speciation.The hypothesis was tested with the millipede Pycnotropis tida CHAMBERLIN, 1941 (Diplopoda: Polydesmida: Aphelidesmidae: Amplininae). Different populations of P. tida that occur in white- and mixedwater inundation forests as well as on terra firme sites were compared with each other and with P. sigma GOLOVATCH, VOHLAND & HOFFMAN, 1998, which only occurs at the terra firme. Comparisons were made on morphological, ethological, chemical, physiological and genetic levels. The transition from terra firme to inundation forest was investigated for P. tida in order to understand adaptation mechanisms and changes in population structure. The center of distribution of the subfamily Amplininae lies in northern South America (nowadays Colombia). Some genera went north as far as the Chiapas in Mexico. The other genera, which share a vesicle like structure at the gonopods as a common feature, developed during the distribution southwards. Most of the Amplininae genera are restricted to higher altitudes of the Andean slope, but Pycnotropis succeeded to inhabit the lower Iying Amazon basin. The distribution of Pycnotropis along the Amazon is caused by passive migration. The animals inhabit logs and reproduce there. Theses logs are washed away from the forest during inundation. As the floodpulse propagates down the river, also the animals are distributed through the Amazon basin. This way they can cover hundreds of kilometers within a few years. In order to determine relationships and species identities of the morphologically very similar species of the genus Pycnotropis allozymelectrophoresis were done at 22 loci as well as sequencing of the satellite family pPeP172 and crossing experiments. The species were indeed reproductively isolated, but cladogenesis occurred only very recently. There are some indications, that the inhabitation of the inundation forest of the Pycnotropis species P. tida P. unapi GOLOVATCH, VOHLAND & HOFFMAN, 1998, and P. madeira GOLOVATCH, VOHLAND & HOFFMAN, 1998 occurred independently. An important selection factor with respect to developmental parameters of f. tida seems to be the duration of the terrestrial phase in the inundation forest. Juvenile individuals do not survive inundation. P. ida needs about seven months to reach adulthood in the laboratory. P. sigma from the non-inundated terra firme needs much more time to develof. The development of f. tida is fast enough to inhabit the inundation forest in higher situated places, where inun-dation due to the flood pulse only lasts five months per year. In lower situated areas with longer inundation periods P. tida is unable to survive. Phenological investigations show that the univoltinie of P. tida in the inundation forest, which was thought to be an adaptation, is only a result of mortality due to the flood pulse. The faster development of f. tida in comparison to P. sigma bears some costs: P. tida reaches only a lower body size than P. sigma. Probably related to this P. tida develops fewer and smaller eggs than P. sigma. Experiments were done to compare P. tida and P. sigma with respect to humidity preference desiccation resistance, and the influence of temperature on egg ripening. From the experi-ments it was concluded that P. tida is only moderately adapted to the abiotic conditions in the inundation forest. Additional comparisons were made with respect to microhabitat and association of P. tida and P. sigma with other wood inhabiting invertebrates on the taxonomic level of the family. Both species inhabit dead wood of low density. There wore no specific associations of the families, neither in the inundation forest nor on the terra firme. This seems to be due to the many processes, which contribute to decomposition of wood and lead to a small scale heterogeneity of the habitat. Aphelidesmidae occur at a late succession stage of wood decomposition. The population density of Aphelidesmidae on the terra firme is much lower than in the inundation forest, which is probably caused by a lower nutrient supply. Laboratory experiments yield an average wood consumption of adult f. tida of 46,5 mg per animal and day, depending on humidity and density of the dead wood. Taking into account the population density and the total amount of dead wood, P. tida consumes only 0.0013% of the annual production of dead wood. The chemical composition of the cuticula (calcium, magnesium, potassium, silicium) of P. tida and P. sigma was analysed using an atomic absorption spectrometer (AAS). P. sigma shows, probably in response to the lower calcium supply of the terra frme compared to the inundation forest, a different composition of the cuticula: the organic fraction is higher and the content of calcium lower. In response to the hypothesis of this thesis it was shown, that the flood pulse most probably is not the determining factor of speciation in P. tida. The species originated only recently in the Andean slope region and adapted to the flood pulse only with respect to its development time. Hence for the millipede of the genus Pycnotropis the importance of the flood pulse lies mostly in the nutrient supply that comes with the seasonal inundations.