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Crosses were made between three closely related species of the planthopper genus Muellerianella, M. brevipennis, M. extrusa and M. fairmairei. Most hybrids are produced in crosses between M. extrusa and M. fairmairei. Most of the hybrid females in these crosses and some of the hybrid males are fertile and can be successfully backcrossed with the parental species. Crosses of M. brevipennis with either M. fairmairei or M. extrusa yield less progeny, of which all males are sterile. Backcrosses of the hybrid females with the parental species are partly successful. The possible evolutionary consequences of mating readiness and interfertility between the three species under field conditions are discussed. Efforts to resynthesize triploid gynogenetic forms by backcrossing hybrid females of M. fairmairei and M. brevipennis with males of M. fairmairei, as was reported by Drosopoulos (Evolution 32: 916–920, 1978), failed. To produce offspring, the existing gynogenetic forms have to mate with males of one of the three bisexual species.

Acoustic behaviour was studied in three closely related planthopper species of the genus Muellerianella (Homoptera, Delphacidae). In order to establish interspecific differences, which are important in reproductive isolation, behavioural interactions between males and females were observed and the associated acoustic signals were recorded and analysed. In addition, geographic variation in male calling songs of two species was studied in populations originating from different parts of Europe. Zusammenfassung Die Laute der Mannchen und Weibchen dreier verwandter Zikadenarten der Gattung Muellerianella wurden aufgenommen und analysiert. Die Arten lassen sich leicht an der Struktur des Rufgesanges der Mannchen unterscheiden. Diese Rufgesange sind wahrscheinlich wichtig fur die reproduktive Isolierung der Arten. Der Werbegesang der Muellerianella-Mannchen unterscheidet sich von dem Rufgesang durch erhohte Intensitat und durch eine Verschiebung im relativen Anteil verschiedener Lautelemente. Der Werbegesang der M. brevipennis-Mannchen ist wesentlich komplizierter wegen mehrerer zusatzlicher Lautelemente. Mannchen aller Arten erzeugen wahrend Auseinandersetzungen vergleichbare Rivalengesange. Die Differenzen und Ahnlichkeiten im akustischen Verhalten fuhren zur Annahme derselben phylogenetischen Beziehungen zwischen den Arten wie fruher morphologische Befunde. Die Rufgesange von M. brevipennis und M. fairmairei, Abkommlinge abgelegener europaischer Populationen, variieren geographisch. Obwohl die Unterschiede im Gebiet der Sympatrie nicht deutlich groser sind als auserhalb, weist die verminderte Variabilitat im Rufgesang von M. fairmairei im Gebiet der Sympatrie auf eine mogliche Interaktion mit M. brevipennis.

The closely related species and forms of the plant-hopper genus Muellerianella have been studied. In Western Europe this complex consists of three bisexual species and a number of triploid forms which reproduce by gynogenesis. M. extrusa Scott 1871 appears to be a distinct species which lives on Molinia caerulea. It is here taken out of synonymy from M. fairmairei Perris 1857, which lives on Holcus lanatus. The third species, M. brevipennis Boheman 1847, lives on Deschampsia cespitosa. Intra- and inter-specific morphological differences in males and females are described in detail, including geographic variation. Triploid forms, which are usually associated with M. fairmairei cannot be distinguished from bisexual females of this species. Distribution maps are given for each bisexual species and for the triploid forms which are based on an evaluation of literature data, museum collections and on 150 new samples taken all over Europe.