br Conclusion br Introduction The morphological variability
Introduction The morphological variability of aquatic plants is well documented, and in some cases has seriously constrained taxonomic resolution (Sculthorpe, 1967, Santamaría, 2002). Many of these variations are caused by the extensive phenotypic plasticity of aquatic plants, but the role of hybridization is also considerable (Les and Philbrick, 1993). Natural hybrids are common in many plant groups (Arnold, 1997), and hybridization has been reported in 35 of 177 (20%) aquatic Adenosine-5\'-diphosphate sale (Les and Philbrick, 1993). Hybridization and introgression have played significant roles in plant speciation, and have resulted in new evolutionarily stable lineages (Stebbins, 1950, Rieseberg et al., 1995). Les and Philbrick (1993) suggested that detailed molecular studies of putative aquatic plant hybrids would reveal a high incidence of morphological variations attributed to hybridization. Molecular markers such as allozymes are powerful tools for detecting the occurrence of hybridization events, and have been used to identify many hybrid species (cf. Arnold et al., 1990). The genetic data can provide successful species-specific markers, through which introgression can be quantified (Arnold et al., 1990, Rieseberg and Ellstrand, 1993). Nuphar Sm. (Nymphaeaceae), the yellow water-lily, is a perennial freshwater, emergent, floating-leaved or submergent macrophyte and is distributed in the temperate zone of the Northern Hemisphere (Beal, 1956, Padgett et al., 1999, Shiga et al., 2006). The genus is one of the most problematic aquatic macrophytes to identify. According to Cook (1990), Padgett (1999), and , 7–20 species have been recognized throughout the world. Natural hybridization and introgression have made determining the species’ delimitation difficult (Beal, 1956, Padgett et al., 1998, Padgett et al., 1999, Padgett et al., 2002, Padgett, 2007, Shiga and Kadono, 2007a). In Japan, five Nuphar species are noted (Kadono, 1994, Shiga et al., 2006): N. japonica DC., N. oguraensis Miki, N. pumila (Timm) DC., N. subintegerrima (Casp.) Makino, N. submersa Shiga & Kadono, with three varieties and two forms. These taxa are endemic to eastern Asia, apart from N. pumila, which is distributed widely in the Old World (Beal, 1956, Kadono, 1994). Kadono (1994) also pointed out the occurrence of intermediate plants. Shiga and Kadono (2004) used morphological data to investigate phenetic relationships of N. japonica, N. subintegerrima, N. oguraensis and intermediate plants in central to western Japan. They distinguished three species, N. japonica, N. oguraensis and N. subintegerrima sensu stricto, and two intermediate plant groups (J-O and J-S groups). Morphological features and geographic distribution patterns of the two intermediate groups suggest that they are of hybrid origin, the J-O group between N. japonica and N. oguraensis and the J-S group between N. japonica and N. subintegerrima. In this study, we conducted allozyme analyses to test the hypothesis that these intermediate plants are of hybrid origin. We focus on the following two questions: (1) Are the two groups of intermediate plants of hybrid origin?, (2) What are the genetic relationships among these Nuphar taxa?
Materials and methods
Discussion Shiga and Kadono, 2004, Shiga and Kadono, 2007a, Shiga and Kadono, 2007b suggested that natural hybridization in Nuphar occurs widely in Japan, and that the two intermediate groups recognized by morphological characteristics are of hybrid origin between N. japonica and N. oguraensis and between N. japonica and N. subintegerrima, respectively. While our genetic evidence supports their hypothesis that the J-S group is of hybrid origin between N. japonica and N. subintegerrima, it indicates that the J-O group is genetically a mixture of three species. Some populations of the J-O group had MLGs with alleles specific to both N. oguraensis and N. subintegerrima. This implies that natural hybridization may have occurred not only between N. japonica and N. oguraensis, but also between N. oguraensis and N. subintegerrima. An additional possibility to be considered is that a hybrid has been formed between N. japonica and N. oguraensis and that a hybrid in turn has crossed with N. subintegerrima.