Neipu , Pingtung , 91201 , Taiwan , Republic of China

The development of new cultivars in Doritaenopsis Guillaum. & Lami orchids is often hindered by factors such as low seed count in hybrids. Cytological study may offer the ability to develop new hybrids by revealing cultivars with good chromosome pairing and high pollen viability, which are somewhat difficult to obtain under current breeding programs. Cross pollination, pollen viability, and chromosomal behavior during meiosis were analyzed to reveal the relation between seed fertility and capsule set in Doritaenopsis hybrids. The number of mature capsules harvested and their relative seed content were used as indices of crossing availability. The results of meiosis were evaluated according to pollen viability detected by fluorescein diacetate and quantification of sporad types by acid fuchsin staining. Chromosome number and pairing at meiosis were observed in root tips or in samples of pollen mother cells. A positive relation was found among high seed set, high frequency of viable tetrads, high degree of chromosome pairing, and low frequency of chromosomal aberrations such as inversions and translocations. On the basis of these factors, three types of hybrids could be distinguished. In type one hybrids, chromosomes paired as bivalents, pollen mother cells divided into tetrads, and capsule setting occurred after pollination of pollen acceptors. In type two hybrids, chromosomes remained mainly as univalents that developed into micromeiocytes, pollen mother cell division was disrupted, and seed recovery was low after pollination. Type three hybrids showed chromosomes paired mostly as multivalents, chromosome bridges, pollen mother cell division with massive failure, and little fertility. In Doritaenopsis orchids, high pollen viability and high fertility depends on a high frequency of normal tetrads, and low seed set in cross-pollination is predicted with micronuclei in the end products of meiosis. The occurrence of chromosomal aberrations may suggest a process of genome differentiation that could compromise breeding efforts if not taken into consideration. In general, relatively little is known about the genetics of orchids because of the high cost of growing large populations of the plant, the relatively long growing cycles ( 6 years) to obtain two generations, and polyploidy in advanced-generation hybrids (Kamemoto et al., 1999). Differences in ploidy are a challenge to the development of new hybrids, and differences in chromosome number may prevent zygote development after pollination (Singh and Hymowitz, 1987). Chromosome doubling is often applied to produce new cultivars of orchids (Lin et al., 2001), as has been the case with colchicine-doubled amphidiploid cultivars in Dendrobium Swartz (Kamemoto et al., 1999). In Phalaenopsis Blume, interspecific and intergeneric hybridization has long been used for breeding superior hybrids, but as for other orchid genera, very little is known about the genetics of the species (Kao et al., 2001). The information currently available is chromosome number (2n = 2x = 38), the karyotype of some representative species, the meiotic chromosome associations in several interspecic hybrids, and the genomic DNA content of several Phalaenopsis and Doritis Lindl. species (Arends, 1970; Lin et al., 2001). Phalaenopsis species may have the same chromosome number, but genome sizes vary considerably (Kao et al., 2001). For instance, Shindo and Kamemoto (1963) compared the mean chromosome length of eight Phalaenopsis species and the closely related species Doritis pulcherrima Lindl. and they found a 2.8-fold difference in chromosome length between species with the largest (P. violacea Witte) and the smallest chromosomes (P. amabilis Blume). Arends (1970) reported two groups in Phalaenopsis with high genome homology distinguished on the basis of chromosomal length at somatic metaphase, as well as chromosomal configurations in metaphase I of hybrids. The first group includes species with small chromosomes, such as P. amabilis, P. aphrodite Rchb. f. , P. equestris (Schauer) Rchb. f. , P. sanderiana Rchb. f. , and P. stuartiana Rchb. f. The second group contains species with large chromosomes, such as P. amboinensis J. J. Sm. and P. mannii Rchb. f. Lin et al. (2001), using flow cytometry and southern hybridization techniques, concluded that chromosome size is Received for publication 6 Aug. 2007. Accepted for publication 6 Oct. 2007. This work was supported by grants from the Council of Agriculture, Taiwan (94AS-1.3.2-FD-Z3 and 95AS-1.3.2-FD-Z3), and by a grant for graduate study to the first author from the International Cooperation and Development Fund of Taiwan, Ministry of Foreign Affairs, Taiwan, Republic of China. We thank Dr. Mei-Chu Chung at the Institute or Plant and Microbial Biology, Academia Sinica, Taiwan, for her critical reading of the manuscript. Corresponding author. E-mail: fchen@mail.npust. edu.tw. J. AMER. SOC. HORT. SCI. 133(1):107–116. 2008. 107 indeed related to repetitive genomic DNA content. Phalaenopsis species such as P. aphrodite, P. sanderiana, P. stuartiana, and P. lueddemaniana Rchb. f. with low DNA values have small-sized chromosomes; those with intermediate DNA values have middlesized chromosomes, and those with high DNA values, such as P. amboinensis and D. pulcherrima, have large-sized chromosomes. The segregation of homologous pairs of chromosomes at meiotic anaphase I depends on prior pairing, synapsis, and recombination at earlier stages (Armstrong and Jones, 2003; Armstrong et al., 2001). Nevertheless, chromosome recognition and initial pairing in eukaryotes is not achieved by DNA–DNA interactions alone (Hiraoka, 1998; Schwarzacher, Table 1. Genome constitution and cytological characteristics of nine Doritaenopsis hybrids. Hybrid Parents (pollen receptor · pollen donor) Length of