Biosystematic studies on the Muellerianella complex (Delphacidae), Homoptera, auchenorrhyncha

The genus Muellerianella comprises the species: M. farmairei, M. brevipennis, M. relicta and one pseudogamous all-female biotype M. fairmairei (3n). The bisexual species M. fairmairei and M. brevipennis as well as the unisexual M. fairmairei (3n) were investigated from a biosystematic point of view. The males of the two bisexual species are morphologically distinct but their females, the female hybrids of both way-crossings between the two bisexual species and the unisexual biotype are morphologically indistinguishable. The bisexual species M. fairmairei and M. brevipennis are diploid (2n = 28) and their sex determination system is XY. F1 hybrids also have 28 chromosomes. The pseudogamous unisexual biotype is triploid (3n = 41) reproducing apomictically, but it requires sperm derived from the males of the two bisexual species to initiate embryogenesis (pseudogamy). The host plants of the Muellerianella complex are Holcuslanatus or H.mollis for the bisexual and unisexual M. fairmairei, and Deschampsia caespitosa for M. brevipennis. The distributions of the two species and their host plants overlap widely in West and Central Europe. M. fairmairei and its host plant are distributed more to the south, while M. brevipennis and its host extend more to the north. There is evidence that the unisexual M. fairmairei (3n) occurs in the overlapping area of the two bisexual species, but is absent from the peripheral areas where one of the other species is also absent. In the area of Leersum-Langbroek (prov. of Utrecht) in Holland both species are common, and their respective host plants grow in reasonable numbers. In a few localities where the two hosts are closely intermixed both delphacid species occur syntopically. Samples of the two species were taken bij the sweep-net and suction methods. M. fairmairei is more frequent in wet biotopes of noncultivated meadows in West Europe. M. brevipennis is more frequent in the north of Europe and is a stenotopic species typical of fresh biotopes of wooded areas. In Northern Europe both species are univoltine while in West and Central Europe M. fairmairei has two distinct generations in contrast to M. brevipennis which has an incomplete second generation. However, there is no important seasonal isolation between the two species. In Southern Europe M. fairmairei is probably polyvoltine. Populations of M. fairmairei from regions where M. brevipennis does not occur (S. Greece, S. France? , Ireland) have a sex ratio of 1 : 1, while populations occurring sympatrically with M. brevipennis (England, France, Holland) have a high proportion of females, comprising a mixture of diploid and triploid individuals. In Holland M. brevipennis has a sex ratio of 1 : 1, while populations of this species in Finland have a high proportion of females. Both diploid species maintained a 1:1 sex ratio in the laboratory rearings. Crossings between triploid females and males of M. fairmairei resulted in absolute all-female triploid progenies. Diploid and triploid females of M. fairmairei coexist in Holland. In one biotope (in Leersum), the proportion of the two female biotypes of M.fairmairei was 1:1, while in others triploid females were more numerous than the diploid ones. It is not clear, whether both female populations of M. fairmairei occupy exactly the same ecological niche. Regarding the wing form of the two bisexual species and the unisexual biotope, long photoperiod (L:D = 18:6) favors the development of the long wings, while short photoperiod (L:D = 10:14) completely suppresses it. Under long photoperiod the macropterous form of M. brevipennis was more common than that of M. fairmairei. Under long photoperiod, when the larval density was increased M. fairmairei (2n) had proportionately more macropterous adults than M. fairmairei (3n). Summer and winter eggs of the two species were parasitized by Anagrus sp. However, eggs of the second generation were more frequently parasitized (up to 40%) that those of the first. The higher population densities of M. fairmairei than of M. brevipennis in the field were interpreted from laboratory observations by the fact that M. fairmairei has higher egg production than M. brevipennis. The triploid females of M. fairmairei are assumed to be more prolific than the diploid ones. Also, the rate of egg production of M. fairmairei was higher than that of M. brevipennis. Females of M. fairmairei mated once produced a few unfertile eggs at the end of their oviposition period, in contrast to M. brevipennis which always produced fertile eggs. Males of M. fairmairei appeared to have greater longevity than the females. During the first generation and in colonies under long photoperiod M. fairmairei oviposits in its food plant H. lanatus, while during the second generation and in colonies under short photoperiod in Juncus effusus. M. brevipennis was found to oviposit in the field only in its food plant, namely D. caespitosa, but in the laboratory it also oviposited in J. effusus. The egg-group size of both species depends upon the oviposition substrate. Embryonic development of both species is continuous during the first generation and in colonies under long photoperiod, but embryonic diapause (arrest of development before blastokinesis) takes place during the second generation and in laboratory rearings under short photoperiod. The intensity of diapause is higher in M. brevipennis as compared to M. fairmairei. Continuous rearings of the unisexual and bisexual M. fairmairei were possible under short photoperiod. The rate of larval development of both species under long photoperiod at 20- 25°C was approximately the same. Under short photoperiod the duration of the last instar larva of females is longer than that under long photoperiod. Temperature, humidity and crowding had an influence upon the rate of larval development. In laboratory experiments, development and reproduction of M. fairmairei occurred on several grass species, in contrast to M. brevipennis which appeared to be monophagous. Mature grasses were more suitable as food plants than seedlings. Some unmated females of M. fairmairei (2n) placed in cages containing H. lanatus and D. caespitosa, and males of M. brevipennis ultimately produced a few male and female hybrids. The reciprocal cross resulted in more female hybrids than males. Egg-fertility of these crosses was variable (0-100%) during the course of the oviposition period. Hybrid larvae can develop on both grass species, but they prefer D. caespitosa to H. lanatus. Males were sterile but females were often fertile and some of them crossed back with M. fairmairei produced a triploid pseudogamous biotype very similar to that collected in the field. The all-female progeny of the triploid M. fairmairei was greater when it was crossed with M. fairmairei males than with M. brevipennis males. Hybrids were obtained even when unmated females (2n + 3n) of both species together were caged with males of one species. In conformity with these results it is proposed that the two bisexual species should be called 'allomorphic-related species' instead of 'sibling species'. Allopatric speciation is considered as the most probable cause of divergence between both species. It is suggested that M. brevipennis originated from an isolated population of M. fairmairei during a period of glaciation and has survived on D. caespitosa. It is proposed to call the unisexual biotype M.2 fairmairei-brevipennis, following the nomenclatorial system of hybrids. In this context, it has been demonstrated that in insects hybridization may lead to unisexuality followed by polyploidy.