Theor. Appl. Genet. 51, 265-270 (1978) ............ ~ "~ ~~ :~'~" s ';~'~i!! 9 by Springer-Verlag 1978 Patemal Regulation of Seed Development in Wheat Hybrids B.S. Gill and J.G. Waines Department of Plant Sciences, University of California, Riverside (USA) Summar~f. Diallel crosses among Triticum boeoticum (4 lines from different geographical areas), T.urartu, Aegilops squcmrosa and Ae. speltoid~8 exhibited reciprocal differences in hybrid seed morphology, endosperm development, and embryo viability. T.~artu and Ae.squarrosa as females with T.boeotic~ and Ae.speltoides lead to shrivelled inviable seed. T.boeotic~m accessions as female with Ae.speltoides also lead to shrivelled seeds. The reciprocal crosses produced plump seeds which either resembled the maternal parent or showed size differences. By altering the endospermic genome ratios, hybrid seeds with I (9)/i (d) showed extreme shrivelling whereas those with 4 (g)/l (d) were medium shrivelled to plump. Genetic experiments involving hybrids of T.boeotic~, T. urartu and T.monococc~ showed that a factor is present in pollen or male gametes, which shows dosage effect and which, by interacting with the maternal genome, leads to endosperm abortion. Key words : Paternal Regulation - Wheat Hybrids - Seed Development - Endosperm Abortion - Genome Unbalance Introduction There are many reports of differences in crossing compatibility and seed development in plant-species hybrids. Pronounced differences in seed morphology and development may be observed between the recip- rocal crosses of two parental lines. In monocotyle- donous plants, reciprocal differences may be mani- fest in one of four classes. I) Endosperm and embryo development may be normal and result in viable seed. Hybrid seed morphology may either resemble the ma- ternal parent or may show size differences. 2) The endosperm may develop abnormally leading to endo- sperm abortion and seed death. Here the embryo it- self is viable but, since it is nursed by the endo- sperm, it dies as a result of the endosperm abortion. 3) Endosperm development is normal, but the hybrid embryo is lethal or semi-lethal. 4) Both the endo- sperm may abort and the embryo may be lethal. We have encountered all of the above reciprocal differences in Triticum L. and Aegilops L. crosses. Boyes and Thompson (1937) studied seed develop- ment in reciprocal interspeeific crosses among 14, 28 and 42 chromosome wheats. The endosperm of "low chromosome" and "high chromosome" crosses exhibited abnormal nuclei, curdy cytoplasm, delayed cellularization and, in some cases, early abortion, whereas the reciprocal crosses underwent normal endosperm and seed development. They attributed this reciprocal difference in seed development to the chro- mosomal unbalance in the endosperm. However, Kiha- ra and Nishiyama (1932) from earlier studies in di- ploid and polyploid Auena crosses had stressed the role of the sperm nucleus in differential endosperm de- velopment. Beaudry (1951)performed similar studies in an Elymus virginicus L. • Agropyron repens (L.) Beauv. hybrid and concluded that A.repens pollen al- ters endosperm development by affecting antipodal cell physiology through a chemical stimulus. His hypothe- sis rests on the assumption that antipodals play a sec- retory role and supply nutrients during early endo- sperm development. Recently, Dhaliwal (1977) showed that cytoplasmic differences cannot account for the re- ciprocal difference between Triticum ur~rtu Turn. and T.boeoticum Boise. crosses, and he has supported the genome unbalance theory of endosperm abortion. Da- vies (1973, 1975) and Davies andBrewster (1975) re- ported on seed size, globulin protein, and ribosomal RNA differences between some reciprocal crosses in Pisum sativum L. They postulated that an inducer in the maternal parent may interact with repressors on the maternal and paternal genomes in the embryo. De- pending on this interaction, maternal and paternal genes may or may not be expressed in the hybrid 0040-5752/78/0051/0265/$1.20