Differences in the synaptic pattern in two autotetraploid cultivars of rye with different quadrivalent frequencies at metaphase I M. Martínez, C. Cuadrado, J. Sybenga, and C. Romero Abstract: Synaptic behaviour of the two tetraploids rye cultivars Gigantón (G) and Tetrapico (T) displaying significant differences in their quadrivalent frequencies at metaphase I was analyzed by electron microscopy in surface-spread prophase I nuclei. A different behaviour was observed between the two cultivars; the synaptonemal complex (SC) quadrivalents frequency being significantly higher in G than in T at prophase I. Moreover, the G SC quadrivalents had more synaptic partner exchanges (SPEs) and their location was more distal than the T SC quadrivalents. However, inverse findings were found at metaphase I, the quadrivalent frequency was higher in T than in G. The role that different factors, mainly the number and location of the SPEs and the frequency and distribution of chiasmata, could play in the evolution from prophase I to metaphase I in both cultivars is discussed. Key words: autotetraploid rye, synaptonemal complex, spreading. Résumé : Le comportement synaptique de deux cultivars du seigle, Gigantón (G) et Tetrapico (T), lesquels montrent des différences significatives au niveau de la fréquence de quadrivalents à la métaphase I, a été examiné en microscopie électronique sur des noyaux étalés en prophase I. Un comportement différent a été observé chez les deux cultivars; la fréquence de quadrivalents à complexe synaptonémal (CS) étant significativement plus élevée chez G que T à la prophase I. De plus, les quadrivalents CS G montraient davantage de changements de partenaires synaptiques (SPE) et leur localisation était plus distale que celle des quadrivalents CS T. Cependant, des observations contraires ont été faites à la métaphase I, la fréquence de quadrivalents étant plus élevée chez T que chez G. Le rôle que certains facteurs, principalement le nombre et l’emplacement des SPE ainsi que la fréquence et la distribution des chiasmes, pourraient jouer dans l’évolution de la prophase I à la métaphase I chez les deux cultivars est discuté. Mots clés : seigle autotétraploïde, complexe synaptonémal, étalement. [Traduit par la Rédaction] Martínez et al. 667 Introduction One of the interesting aspects of polyploids and poly- somics lies in the fact that more than two potential homolo- gous partners are available for pairing, giving the possibility of studying meiotic behaviour in competitive situations. Metaphase I studies of autotetraploid grasses revealed an un- expectedly high bivalent frequency in most cases (reviewed by Kuspira et al. 1985). Several mathematical approaches were postulated in order to establish quadrivalent frequency at pachytene from the more accessible data at metaphase I (Sybenga 1975; Hamey et al. 1988). These models were based on the relative frequencies of different metaphase I configurations. The development of surface-spreading tech- niques for making whole-mount preparations of synaptonemal complexes (SCs) has allowed the direct observation of a higher number of prophase I nuclei in polyploid materials (Gillies 1989). These studies were used to quantitatively an- alyze the meiotic synaptic process directly, making clear that the aforementioned models are poor estimators, however useful, of prophase I synaptic behaviour, because of the large discrepancy between the indirectly estimated values and the value of direct prophase I observations (Gillies et al. 1987; Jones and Vincent 1994). So, the causes of a reduced quadrivalent frequency at metaphase I of polyploids must be analyzed using direct zygotene–pachytene observations. From a previous paper on autopolyploids of rye (triploids and tetraploids), it has been established that prophase I of rye tetraploid differs from other tetraploids and is character- ized by: (i) weak alignment of the four unsynapsed or par- tially synapsed homologous axes; (ii) low number of synaptic partner exchanges (SPEs) displayed by SC quadri- valents; and (iii) the existence of SC multivalents more com- plex than quadrivalents as a result of non-homologous chromosomal associations (Santos et al. 1995). Autotetraploid rye cultivars were known to show different frequencies of quadrivalents at metaphase I. In the present paper two tetraploid cultivars of rye displaying a very differ- ent quadrivalent frequency at metaphase I were studied in order to investigate the relationship between the synaptic pattern, especially the number and location of SPEs at Genome 42: 662–667 (1999) © 1999 NRC Canada 662 Corresponding Editor: C. Gillies Received August 5, 1998. Accepted January 12, 1999. M. Martínez, C. Cuadrado, and C. Romero. 1 Departamento de Genética, Facultad de Ciencias Biológicas, Universidad Complutense, Madrid 28040, Spain. J. Sybenga. Department of Genetics, Wageningen Agricultural University, Dreijenlaan 2, 6703 HA Wageningen, The Netherlands. 1 Author to whom all correspondence should be addressed.