Experientia 42 (1986), Birkh~iuser Verlag, CH 4010 Basel/Switzerland 803 Fedoroff and Hertz. Academic Press, New York 1978. 34 Stedman, T. L., Medical dictionary. The Williams and Wilkins Com- pany, Baltimore 1977. 35 Steinv~g, S. K., and Laerum, O. D., Fetal rat brain hemisphere tissue in nonadherent stationary organ culture. Experientia 41 ( 1985) 1517- 1524. 36 The Boulder Committee, Embryonic vertebrate central nervous sys- tem: Revised terminology. Anat. Rec. 166 (1970) 257-262. 37 Trapp, B.D., Honegger, P., Richelson, E., and Webster, H. deF., Morphological differentiation of mechanically dissociated fetal rat brain in aggregating cell cultures. Brain Res. 160 (1979) 117-130. 38 Varon, S., and Rainborn, C.W., Fractionation and culture of em- bryonic brain cells. Brain Res. 12 (1969) 180-199. 0014-4754/86/070798-0651.50 + 0.20/0 9 Birkh~iuser Verlag Basel, 1986 Mini-Review Non-reciprocal fertility among species of the Aedes (Stegomyia) scutellaris group (Diptera: Culicidae) by Vas Dev* Department of Biology, University of Notre Dame, Notre Dame (Indiana 46556, USA) Summary. The genetic basis of non-reciprocal fertility in the Aedes scutellaris group has been viewed in relation to cytoplasmic incompatibility in the Culex pipiens complex and related to the presence or absence of rickettsia-like microorganisms Wolbachia spp. A crossing scheme is proposed to explore the genetic basis of non-reciprocal fertility between two species by crosses involving a third mutually compatible species. Key words. Cytoplasmic incompatibility; Aedes scutellaris ; Wolbachia ; speciation. There has been considerable interest in the phenomenon of non-reciprocal fertility in mosquitoes because of its potential use in population suppression programs. Also, the mechanism(s) underlying non-reciprocal fertility may be important for an understanding of speciation. This phenomenon is widespread in the family Culicidae; an increasing number of species complexes are being recognized and crossing relationships between sibling species are at present being studied. Non-reciprocal fertil- ity in the Aedes seutellaris group was first reported to occur between Ae.scutellaris scutellaris Walker and Ae. seutellaris katherinensis Woodhill 1. Fertile hybrids were obtained by crossing Ae. s. scutellaris females with Ae. s. katherinensis males; in the reciprocal cross Ae. s. katheri- nensis females were inseminated by Ae. s. scutelIaris males but all eggs laid were inviable. Woodhill 2 further showed that the Ae.s.katherinensis females when crossed with hybrid males derived from the cross scutellaris fe- males x katherinensis males laid inviable eggs; the other three backcrosses were successful. The origin and nature of such non-reciprocal fertility is of genetic interest and can be analysed using egg hatch as a marker since viable hybrids can be obtained in one direction and the hybrids can be backcrossed to parental species. Smith-White 3 drew attention to possible genetic mecha- nisms underlying this phenomenon of non-reciprocal fer- tility and suggested a sequential backcrossing scheme which was later exploited by Laven4 studying the Culex pipiens complex. Smith-White and WoodhilP, studying ,the Ae.scutellaris complex, used this backcrossing scheme to determine the possible genetic bases for non- reciprocal fertility. The latter authors concluded that non-reciprocal fertility must depend either on anomalous meiosis in oogenesis or on nucleus-independent cytoplas- mic factors. The second mechanism paralleled the find- ings of Laven 4'6, based on his extensive sequential back- crossing experiments with the C.pipiens complex. Since this phenomenon of non-reciprocal fertility or unidirec- tional incompatibility appears to have a cytoplasmic basis, this is often referred to as cytoplasmic incompati- bility, and these terms are used interchangeably. McClel- land 7 has proposed an alternative to the hypothesis of cytoplasmic inheritance to account for the phenomenon of non-reciprocal fertility, reciprocal fertility and recip- rocal incompatibility, which involves cytoplasmic condi- tioning alleles and preferential segregation. Yen and Barrs attributed this cytoplasmic incompatibil- ity in the C.pipiens complex to the presence of rickettsia- like organisms, WoIbachia spp. Later they demonstrated that the incompatibility barrier could be broken by removing these microorganisms by a treatment of larvae with antibiotics, e.g. tetracycline hydrochloride9. Fur- ther, Barr 1~ offered a hypothesis to account for strain differences affecting fertility and multiplicity of crossing types in C.pipiens based on the presence of different kinds of symbionts and their ability to evolve with their hosts. These microorganisms have also been reported in species of the Ae. scutellaris group 11-14; further, incompa- tible crosses have been known to become compatible after heat or antibiotic treatment 15. However, several questions raised by Trpis et al. is, and Subbarao 16 remain unanswered and the mechanism of non-reciprocal fertil- ity remains unsettled. Furthermore, for some crosses both antibiotic and heat treatments failed to produce compatibility17,18; in other crosses the presence or absence of Wolbachia seemed to be in conflict with the crossing