Ecological differentiation and reproductive isolation of two closely related sympatric species of Oenanthe (Apiaceae)

Oenanthe conioides is a lower Elbe endemic plant species growing in the freshwater tidal zone around Hamburg (Germany). Its closest relative Oenanthe aquatica is widely distributed in Eurasia and grows in calm and shallow freshwater. The two species differ in habitat requirements but are otherwise sympatrically distributed, suggesting that ecological divergence has to be maintained in the face of gene flow. In the present study, we investigated ecological differentiation and reproductive isolation in these two species. An amplified fragment length polymorphism analysis found clear genetic differentiation between the two species implying reproductive isolation. A reciprocal transplantation experiment including artificial F₁ hybrids showed strong selection against immigrants. In the two parental habitats, the non‐native species are less fit than the native species. Hybrids are less fit in the habitat of Oe. aquatica but perform as well as the native species in the habitat of Oe. conioides. We hypothesize that selection against immigrants is the most important component of reproductive isolation between the two species, and that selection against immigrants is the result of exposure to cold and wind in the tidal habitat of Oe. conioides and to herbivory in the habitat of Oe. aquatica. These results are congruent with a role for differential ecological selection in the formation and maintenance of these two species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 526–535.     

Structure, patterns and function of cuticular terraces in trilobites

Schmalfuss, Helmut 1981 12 15: Structure, patterns and function of cuticular terraces in trilobites [Konstruktionsmorphologie Nr. 1331. Lethaia . Vol. 14, pp. 331–341. Oslo. ISSN 0024–1164.
Cuticular terrace systems are widespread in trilobites. Two different patterns, occurring independently of each other, are recognized: transverse dorsal terraces with the scarp facing backwards, and concentric ventral terraces on the doublures with the scarp facing outwards. Ontogenetically, there is a secondary increase of terrace number, so distances between single terraces remain constant. The dorsal terraces functioned to provide unidirectional friction facilitating the burrowing process. The ventral terraces on the doublures were used to consolidate the walls of a filter chamber underneath the animal. * Trilobita, cuticular terraces, functionol morphology .