Abstract We recently showed that aposporous apomix- is, a form of gametophytic apomixis, is controlled by a single apospory-specific genomic region (ASGR) in both Pennisetum squamulatum and Cenchrus ciliaris. We present evidence that in a reciprocal cross between sexu- al pearl millet (P. glaucum) and an apomictic F1 (P. glaucum × P. squamulatum) the ASGR is not transmitted at the same rate. When pearl millet was used as the female parent and the apomictic genotype as the pollen donor, the ASGR was transmitted at a rate of 0.41 in a progeny of 57 plants, indicating a slight transmission ra- tio distortion. However, in a population of 52 rare sexual progenies characterized among a large progeny of a qua- si-obligate apomict (an F1 hybrid of P. glaucum × P. squamulatum), the transmission rate of ASGR was only 0.12. This strong segregation distortion may have occurred at four different levels: (1) female meiosis, (2) during female gametophyte maturation, (3) upon fertili- zation with differential survival of embryos being a con- sequence of differential gene expression controlled by parent-of-origin specific effects (imprinting) and (4) at a later developmental stage of the embryo through an embryo/endosperm genetic incompatibility system. Keywords Apomixis · Apospory · Segregation ratio distortion · Gamete lethality · Imprinting Introduction Apomixis is a genetically inherited trait by which plants are able to clone themselves through seeds without in- volvement of meiosis or fertilization of the egg (Asker and Jerling 1992). Apospory is the most common form of gametophytic apomixis in grasses (Brown and Emery 1958). Unreduced nucellar cells develop into ameiotic embryo sacs while the sexual pathway of megasporogen- esis and development of reduced Polygonum-type em- bryo sacs abort (Nogler 1984). The unreduced embryo sacs in most aposporous grasses differentiate four nuclei with no antipodal cells, a feature easily identified through cytological observations. The unreduced egg cell develops parthenogenetically into an embryo. How- ever, in some apomicts fertilization of the central cell re- mains a requirement in order to achieve formation of the endosperm and assure further development of the em- bryo (Nogler 1984), a process called pseudogamy. We recently demonstrated that apospory is under the dominant control of an apospory-specific genomic re- gion (ASGR) conserved in at least two grass species, Pennisetum squamulatum and Cenchrus ciliaris (Ozias- Akins et al. 1998; Roche et al. 1999). The ASGR is de- fined as a linkage group of several molecular markers for which virtually no recombination has been observed in either species. In a large progeny of 397 plants ob- tained from the cross between tetraploid pearl millet (P. glaucum) and hexaploid P. squamulatum, in which the latter was used as the pollen donor, we observed that the ASGR followed a tetrasomic inheritance with signifi- cant negative segregation distortion (Ozias-Akins et al. 1998). The nature of this distortion remains uncharacter- ized. The question arises whether the same phenomenon occurs during female meiosis. Insights on the transmis- sion of ASGR in the female parent may shed light on some of the elements of apomixis, which seem to affect only female meiosis and subsequent development of em- bryo sacs. Evaluation of the transmission of the ASGR through female meiosis, however, is complicated due to the dominant nature of this DNA region. Nearly complete penetrance of the apomictic trait limits the number of sexually-derived progenies to investigate. Only extensive crosses with a pollen donor carrying a dominant marker which facilitates early selection of scarce sexually- D. Roche · W.W. Hanna U.S. Department of Agriculture, Agricultural Research Service, Costal Plain Experiment Station, Tifton, GA 31793-0748, USA P. Ozias-Akins ( ✉ ) · Z. Chen Department of Horticulture, University of Georgia, Coastal Plain Experiment Station, Tifton, GA 31793–0748, USA e-mail: ozias@tifton.cpes.peachnet.edu Fax: +1–912–386–3356 Sex Plant Reprod (2001) 13:217–223 © Springer-Verlag 2001 ORIGINAL ARTICLE Dominique Roche · Zhenbang Chen Wayne W. Hanna · Peggy Ozias-Akins Non-Mendelian transmission of an apospory-specific genomic region in a reciprocal cross between sexual pearl millet (Pennisetum glaucum) and an apomictic F1 (P. glaucum × P. squamulatum) Recevied: 13 June 2000 / Revision accepted: 23 October, 2000