Description of a new natural Sonneratia hybrid from Hainan Island, China

Abstract Here, we describe, illustrate and compare a new natural hybrid, Sonneratia × zhongcairongii Y. S. Wang & S. H. Shi (Sonneratiaceae), with its possible parent species. Based on its morphological characteristics and habitat conditions, this taxon is considered to represent a sterile hybrid between S. alba and S. apetala. In China, the new hybrid is only reported in the mangrove forest in Dongzhai Harbour, Hainan Island. It has intermediate characteristics with its parents by elliptical leaf blades, peltate stigma, terminal or axillary inflorescence with 1–3 flower dichasia, cup – shaped calyx (4–6 calyx lobes) and no petals. We also provide a key for the identification of Sonneratia species.


Introduction
Sonneratiaceae is a small tropical plant family consisting of only two small genera, Sonneratia and Duabanga. The inland genus Duabanga is an evergreen component of the rainforest belt, comprising two species within a more restricted range in Malaysian, Indonesia and China (Tomlinson 1986;Goutham-Bharathi et al. 2012). Sonneratia, a genus of mangroves, is one of the most important components of the intertidal zones of the tropical and subtropical coastal regions, ranging from East Africa through Indo-Malaya to tropical Australia and into Micronesia and Melanesia (Tomlinson 1986). This genus is also well-adapted to harsh intertidal zones with high salinity, hypoxia and ultraviolet (UV) radiation (Duke et al. 1998).
Sonneratia consists of six species and three interspecific hybrids (Duke and Jackes 1987;Duke 1994;Goutham-Bharathi et al. 2012;Yang et al. 2016). Amongst these, S. alba, S. caseolaris, S. ovata and S. × gulngai are the most widespread species (Tomlinson 1986;Goutham-Bharathi et al. 2012;Yang et al. 2016), whereas S. lanceolata and S. × urama are strictly limited to north-western Australia, southern New Guinea and a few locations in Indonesia (Yang et al. 2016). S. griffithii has a restricted distribution along the shores of the Andaman Sea, north to Bengal and south to the upper Malay Peninsula (Tomlinson 1986). S. × hainanensis, a hybrid derived from the cross between S. alba and S. ovata, is found in Hainan, China (Ko 1985;Wang et al. 1999). It was first reported that S. × hainanensis was in north-western Borneo as a nom. nud., based on morphological and cytological analyses (Muller and Hou-Liu 1966;Zhou et al. 2005). The parents of S. × hainanensis are widely distributed; however, more collections are needed. The mangrove S. apetala is restricted to southern India and Burma and is the most distinctive species because of its mushroom-like stigma (Tomlinson 1986). In China, S. apetala was first introduced in Dongzai Harbour, Hainan Island in 1985 from Bangladesh (Peng et al. 2012). Due to its accelerated growth and high tolerance of environmental stresses, S. apetala has been used as the pioneer species for mangrove restoration in estuarine and coastal areas. The species, S. alba, is an endemic species on Hainan Island (Li et al. 2017). Between two mixed populations, plants with intermediate characteristics have recently been encountered that obviously belong to the hybrid. In this study, we describe the new hybrid, S. × zhongcairongii and its features that distinguish it from both parent species.

Materials and method
The morphology of Sonneratia species presented here is based on field, vegetative and reproductive characteristics. Field traits were recorded on site, whereas vegetative and reproductive characteristics were observed and measured using fresh specimens, material preserved in 70% ethanol or press-dried specimens. Digital calipers (Mitutoyo, Japan) and a dissecting microscope with calibrated eye (Olympus, Germany) were used to describe the detailed morphological characteristics of samples. All photographs were taken in the field, i.e. in the natural habitat of the species, using a digital camera (cannon EOS RP, Japan). The morphological characteristics of Sonneratia species in Hainan Island have been summarized in a key to facilitate identification.

Discussion
To date, only three hybrids including S. × zhongcairongii have been reported in the genus Sonneratia. As with S. × zhongcairongii, other two hybrids have restricted location in the cross distribution of each parents (Duke and Jackes 1987;Duke 1994;Goutham-Bharathi et al. 2012;Yang et al. 2016). Only two individuals of the new hybrid were observed in China. The parent, S. apetala, is an exotic species in China, whose mixed location with S. alba started from 1985 (Peng et al. 2012). The morphological characteristics of S. × zhongcairongii were intermediate between its parents (Figs 2, 3); this result is consistent with the other two Sonneratia hybrids (Tomlinson 1986;Duke and Jackes 1987). S. × zhongcairongii showed complete abortion. However, on the other two hybrids (S. × gulngai and S. × hainanensis) can be found fruit and seeds with heavy abortion degrees (Tomlinson 1986;Wang and Wang 2007). Backer and van Steenis (1951) compiled a thorough review of the Sonneratiaceae, a family of the order Myrtales. Two genera were described and include Duabanga and sonneratia. Gao Yunzhang divided the Sonneratia genus into two sections, sect. Sonneratia and sect. Pseudosonneratia, based on the presence or absence of petals (Ko 1985) and which was also used in the research of sonneratia Linn. in Australia, New Guinea and the south-western Pacific region (Backer and van Steenis 1951). By adding one new species found in China (S. paracaseolaris Ko, E. Y. Chen et W. Y. Chen), Gao Yunzhang regrouped the Sonneratia species in China (Ko 1993). Subsequently, a detailed anatomical analysis containing morphology of leaf, flower, fruit, seed and wood of five species of Sonneratia Linn. in China showed that the use of petal presence or absence was appropriate to distinguish species in Sonneratia Linn. (Chen 1996). Duke and Jackes worried about the use of petal presence or absence to distinguish between apetalous S. alba with S. ovata which was found to be less common, normally apetalous (Duke and Jackes 1987). Then the wrong character of S. alba was revised from apetalous to white, linear-spathulate (Wang and Wang 2007). Compared with characters of petal, stamen, leaf and flower bud, the stigma morphological characteristics have been used to group nine species and hybrids in Sonneratia Linn. (Wang and Chen 2002). In this study, we combined the use of petal presence or absence and stigma morphological characteristics to regroup Sonneratia plants and the new hybrid was most closely related to one of its parents, S. apetala.
To better distinguish amongst species belonging to the genus Sonneratia, we created a classification as shown in Table 1. The distribution range of the hybrid S. × zhongcairongii often overlaps with that of S. alba Smith. and S. apetala Buch. -Ham., which provides the possibility of formation of the hybrid species. The same is true for S. × gulngai N. C. Duke, S. × hainanensis Ko, E. Y. Chen et W. Y. Chen (Wang et al. 1999). The overlapping distributions of parent species contributed to the greater opportunity to form a natural hybrid (Zhou et al. 2008). Interestingly, one of the parents of all three hybrids is S. alba, which may be due to the fact that S. alba is a widely-distributed species, although further investigation is needed to determine the exact reason.