15 Interspecific Hybridization in Lily (Lilium): Interploidy Crosses Involving Interspecific F 1 Hybrids and Their Progenies Shujun Zhou 1,2  Rodrigo Barba-Gonzalez 3  Ki-Byung Lim 4  M.S. Ramanna 1  Jaap M. Van Tuyl 1* 1 Plant Research International, Wageningen University and Research Center, Droevendaalsesteeg 1, 6708 PB Wageningen, The Netherlands 2 College of Agriculture and Biotechnology, Zhejiang University, Hangzhou, Zhejiang Province, 310029 China 3 Centro de Investigación y Asistencia en Tecnología y Diseño del Estado de Jalisco A.C. Biotecnología Vegetal, Av. Normalistas #800. Colinas de la Normal., Guadalajara, Jalisco, C.P. 44270, Mexico 4 School of Applied Biosciences, College of Agriculture & Life Sciences, Kyungpook National University, Daegu 702-701, Republic of Korea Corresponding author: * jaap.vantuyl@wur.nl Keywords: embryo, endosperm ploidy level, genomic in situ hybridization, pseudoeuploids ABSTRACT Despite a long history of cultivation, a large number of lily (Lilium) cultivars were still diploid (2n=2x=24) till recently. But polyploid cultivars are rapidly increasing. In addition to having robust stems, large flowers, thicker and larger leaves, polyploids can also serve to combine desirable characters from species of different taxonomic sections; not only from the cultivated groups, viz., Sinomartagon, Archelirion and Leucolirion, but also from sections that include non-cultivated species. Because the F1 hybrids between the species of different taxonomic sections are highly sterile, it is imperative that breeding has to be carried out at the polyploidy level. This chapter is based on our results on using intersectional interspecific hybrids of Longiflorum x Asiatic (LA) and Oriental x Asiatic (OA) groups of lilies and their polyploid backcross progenies BC1, BC2 and BC3). In order to cross genotypes of different ploidy levels, i.e., interploidy crosses, a knowledge of embryo and endosperm ploidy levels is valuable because, unlike in most other plant species, lily has very large chromosomes and the embryo sac formation is of tetrasporic 8-nucleate type. In order to highlight the differences, a comparison is made with the most commonly occurring, monosporic 8-nucleate type of diploid potato, with 24 chromosomes (same as lily). Some of the limitations and constraints associated with interploidy crossing are considered. 1. INTRODUCTION Generally, polyploid plants differ from their diploid counterparts in various ways. As a result of increase of chromosome numbers as well as DNA content, polyploids may possess robust stems, larger flowers, thicker leaves and can be more attractive in appearance in some cases. Besides these, some physiological features such as flower initiation, duration of flowering, altered water relations and photosynthesis may also be affected (for review, see Ramsey and Schemske 2002). Therefore, not surprisingly, horticultural plant breeders have selected polyploid forms in numerous ornamental plants in order to create new varieties (Darlington 1967; Van Tuyl et al. 2002). Although lilies (Lilium) have been cultivated for several centuries (Evans 1921, 1930; Woodstock and Stearn 1950), a large number of cultivars were probably diploids (2n=2x=24) till recently. At present, however, a large majority (80% or more) of cultivars are polyploids. Not all polyploid cultivars of lilies have been listed so far. However, in Asiatic group of lily hybrids, more than 120 polyploid cultivars were listed and these include triploids (2n=3x=36), tetraploids (2n=4x=48) as well as a few aneuploids (Schmitzer 1991). In addition to these autopolyploids, allopolyploid lily cultivars are becoming increasingly popular. These include interspecific polyploids involving Longiflorum x Asiatic hybrids (LA), Longiflorum x Oriental hybrids (LO), Oriental x Asiatic hybrids (OA) and Oriental x Trumpt lily hybrids (OT). Recent statistics indicate that the afore-mentioned groups of allopolyploids occupy nearly a quarter of the area used for lily bulb production in the Netherlands (Chapter by Lim et al. in this series). Although no critical data on the chromosome numbers of all allopolyploid lily cultivars are yet available, it appears that both triploid and tetraploid forms are being cultivated. A recent survey of chromosome constitutions in a number of Dutch lily cultivars has shown that allotriploids of interspecific origin, involving Longiflorum and Asiatic lilies, are predominant among them (Zhou 2007). Despite the recognition of the presence of both auto- and allopolyploid cultivars of lilies, little is known about the exact origins of most of the polyploid cultivars, with the exception of a few cultivars for which detailed information on the parentage and the chromosome constitution are available (Van Tuyl et al. 2002; Zhou 2007). In these cases, both somatic doubling of chromosomes by colchicine or oryzalin as well as sexual polyploidization, (i.e., induced by unreduced (2n) gametes) have been used (Van Tuyl 1989). ®