Chromosome pairing in the allotetraploid Aegilops biuncialis and a triploid intergeneric hybrid N. Cuiiado, S. Callejas, M.J. Garcia, A. Fernandez, and J.L Santos Abstract: Chromosome pairing behaviour of the natural allotetraploid Aegilops biuncialis (genome UUMM) and a triploid hybrid Ae. biuncialis X Secale cereale (genome UMR) was analyzed by electron microscopy in surface-spread prophase I nuclei. Synaptonemal-complex analysis at zygotene and pachytene revealed that synapsis in the allotetraploid was mostly between homologous chromosomes, although a few quadrivalents were also formed. Only homologous bivalents were observed at metaphase I. In contrast, homoeologous and heterologous chromosome associations were common at prophase I and metaphase I of the triploid hybrid. It is concluded that the mechanism controlling bivalent formation in Ae. biuncialis acts mainly at zygotene by restricting pairing to homologous chromosomes, but also acts at pachytene by preventing chiasma formation in the homoeologous associations. In the hybrid the mechanism fails at both stages. Key words: Aegilops biuncialis, allotetraploid, intergeneric hybrid, pairing control, synaptonemal complex. Rbsumb : L'appariement chromosomique chez l'allotktraploi'de nature1 Aegilops biuncialis (le gknome UUMM) et chez un hybride triploi'de Ae. biuncialis X Secale cereale (le gknome CTMR) a kt6 examink par microscopic klectronique sur des ktalements de noyau en prophase I. Une analyse du complexe synaptonkmal aux stades zygotkne et pachytkne a rkvklk que la synapsis chez l'allotktraploi'de avait lieu surtout entre chromosomes homologues bien que quelques quadrivalents aient kgalement kt6 observks. Seuls des bivalents ont kt6 observks h la mktaphase I. Au contraire, des associations entre chromosomes hom6ologues et hktkrologues ktaient rkpandues chez l'hybride triploi'de lors de la prophase I et de la mktaphase I. Les auteurs en concluent que le mkcanisme rkgissant la formation des bivalents chez le Ae. biuncialis agit principalement lors du stade zygotkne en limitant l'appariement aux seuls chromosomes homologues mais kgalement lors du pachytkne en prkvenant la formation de chiasmes entre chromosomes homkologues appariks. Chez I'hybride, ce mkcanisme fait dkfaut lors de ces deux stades. Mots elks : Aegilops biuncialis, allotktraploi'de, hybride intergknkrique, contrale de l'appariement, complexe synaptonkmal. [Traduit par la Rkdaction] Introduction Allopolyploidy is a widespread phenomenon in flowering plants having been observed in genera as different as Nicotiana, Brassica, Gossypium, Spartina, Galeopsis, and Triticum (Stebbins 197 1). Allopolyploids usually arise by chromosome doubling following hybridization, therefore, they contain several structurally and genetically similar chromosome sets with the capacity for pairing and crossing- over. Despite this, they behave as diploids, owing to the almost exclusive presence of homologous bivalents at first metaphase. The development of surface-spreading techniques for making whole mount preparations of synaptonemal Corresponding Editor: C.B. Gillies. Received November 28, 1995. Accepted April 4, 1996. N. Cuiiado, S. Callejas, M.J. Garcia, and J.L. Santos. Departamento de Genktica, Facultad de Biologia, Universidad Complutense, 28040 Madrid, Spain. A. Fernandez. Centro de Microscopia Electrbnica, Universidad Complutense, 28040 Madrid, Spain. complexes (SCs) in recent years has permitted a more accurate knowledge of chromosome behaviour at early prophase I stages. These techniques have also been applied to the analysis of the chromosome pairing process in some allopolyploid cereals and grasses, namely: Triticum aestivum (Holm 1986; Holm and Wang 1988), 7: aestivum X Triticum kotschyi hybrids (Gillies 1987), Lotus corniculatus (Davies et al. 1990), and Festuca arundinacea and Festuca gigantea (Thomas and Thomas 1993). From these studies it can be concluded that diploidization is the product of several processes operating at different times throughout meiotic prophase: (i) chromosome pairing initiated preferentially between homologues at the beginning of zygotene, (ii) transformation of multiple associations into bivalents at the end of zygotene and during pachytene before crossing- over occurs, and (iii) supression of chiasma formation between paired homoeologous segments. The relative importance and precision of these constituent processes varies between species. Since the main interest in the allopoiyploid field has been focused on wheat and its relatives, the lack of infor- mation on chromosome pairing of polyploid Aegilops Genome, 39: 664-670 (1996). Printed in Canada 1 ImprimC au Canada Genome Downloaded from www.nrcresearchpress.com by Renmin University of China on 06/07/13 For personal use only.