Synotis xinningensis (Asteraceae), a new species from Hunan, China

Background Synotis is one of the several genera within Senecioneae (Asteraceae) with more than 40 species that are mainly distributed in China or in the Sino-Himalayan region. During a botanical expedition in central and southwestern China in 2011, we found an unusual population of Synotis in southwestern Hunan Province. To determine the taxonomic identity of the population, we carried out gross-morphological, floral micromorphological, and cytological observations. Results Our gross-morphological observations have shown that the population is most similar to Synotis changiana Y. L. Chen, but readily distinguishable in the discoid capitula (vs. radiate), and in the bracts of calyculus 9–10 (vs. 6–8), 6–7 mm long (vs. 3–4 mm). The floral micromorphological observations on the population and S. changiana agree with previous reports for other species of Synotis. The chromosomes of the population are counted to be 2n = 40 + 0–1B. Its karyotype is formulated as 2n = 22m + 14sm + 4st. Conclusions The population is determined to represent a new species, i.e. Synotis xinningensis M. Tang & Q. E. Yang, which is described herein. The new species belongs to Synotis ser. Synotis. Electronic supplementary material The online version of this article (doi:10.1186/1999-3110-54-16) contains supplementary material, which is available to authorized users.


Background
Within Senecioneae (Asteraceae), there are several genera with more than 40 species that are mainly in China or in the Sino-Himalayan region. They include Cremanthodium Benth., Ligularia Cass., Parasenecio W. W. Smith & Small, Sinosenecio B. Nord., and Synotis (C. B. Clarke) C. Jeffrey & Y. L. Chen (Jeffrey and Chen 1984;Chen et al. 2011). One of us (Yang) and colleagues have a strong interest in these genera and are carrying out in-depth systematic studies of them. These studies have focused on Sinosenecio (e.g., , 2011a, b, c, d, 2012Liu et al. 2009Liu et al. , 2011Zhang et al. 2008) and have made important contributions to our knowledge of the genus (Nordenstam and Pelser 2011). The genus now under study is Synotis.
Synotis is a genus of about 54 species, all endemic to the Sino-Himalayan region except for S. atractylidifolia (Y. Ling) C. Jeffrey & Y. L. Chen, which occurs in northern China (Jeffrey and Chen 1984;Chen 1999;Nordenstam 2007;Chen et al. 2011). Chen (1999) and Chen et al. (2011) recorded 43 species of Synotis in China.
During a botanical expedition in central and southwestern China in 2011, we found an unusual population of Synotis on Lang Shan, Xinning County, in southwestern Hunan Province. The plants were most similar to S. changiana Y. L. Chen, a species described from Guangxi (Chen 1995), but very readily distinguishable in the discoid capitula. Further examination revealed additional differences in the number and length of the bracts of calyculus. We determined that the population represents an undescribed species, i.e. Synotis xinningensis M. Tang & Q. E. Yang, which is here described. In 2012, we found two additional gatherings of the new species in the Herbarium of the Institute of Botany, Chinese Academy of Sciences (PE).

Floral micromorphological character observations
For observation of the filament collar and anther endothecial cell wall thickenings of Synotis xinningensis (voucher: Long-yuan Wang & Ming Tang 123, HAST, IBSC) and its putative closest relative, S. changiana (voucher: Long-yuan Wang & Ming Tang 111, HAST, IBSC), heads were boiled in distilled water for 3 min, and then fixed in Carnoy's solution (glacial acetic acid: absolute ethanol = 1: 3). Mature disc florets removed from the fixed heads were dehydrated in 70% ethanol for 30 min, then placed in 99% ethanol for 1 h before they were treated with 5% NaOH overnight. The anther tissue was isolated from the florets on the slide, flooded with 50% glycerol and a cover slip was applied. Samples were then examined at 100× (filament collar) and 400× (endothecial cell wall thickenings) magnification by light microscopy and photographed.

Etymology
The specific epithet 'xinningensis' is derived from the type locality, Xinning County, southwestern Hunan Province, China.

Distribution and habitat
Synotis xinningensis is currently known only from Xinning County, southwestern Hunan Province, China (Figure 3).
It grows in open mixed forests at 450-1,000 m above sea level.

Floral micromorphological characters
As shown in Figure 4A, C, the filament collars of both Synotis xinningensis and S. changiana are balusterform, being basally dilated and consisting of larger cells, conforming to the results reported previously for some other species of Synotis (Jeffrey and Chen 1984). The anther endothecial cell wall thickenings of S. xinningensis ( Figure 4B) were distributed along all the inner walls of the endothecial cells, and thus were radial. Synotis changiana also had the same pattern of anther endothecial cell wall thickenings ( Figure 4D). The observations agree with previous reports for other species of Synotis (Jeffrey and Chen 1984). The floral micromorphological characters indicate that Synotis is a member of subtribe Senecioninae as defined by Nordenstam (2007).

Chromosome cytology
The metaphase chromosomes of Synotis xinningensis were counted to be 2n = 40 + 0-1B ( Figure 5A). According to the chromosome nomenclature of Levan et al. (1964) 20m + 16sm + 4st. No B-chromosomes were observed in this species. In chromosome number and chromosome morphology, Synotis xinningensis is essentially the same as S. changiana.
The genus Synotis is poorly known cytologically, with reports of chromosome numbers for only three species. Synotis alata (Wall. ex DC.) C. Jeffrey & Y. L. Chen was reported from Indian material, under the name Senecio alatus Wall. ex DC., to have n = 20 (Mehra et al. 1965). The chromosomes of S. rufinervis (DC.) C. Jeffrey & Y. L. Chen were reported, also from Indian material and under the name Senecio rufinervis DC., to have n = 18 (Mehra and Remanandan 1975), n = 10 ( Gill 1981, 1989;Gupta et al. 2010), or n = 20 (Gupta and Gill 1989;Gupta et al. 2010). Liu et al. (2006)  It should be noted that in that paper the plant was stated to have radiate capitula, but S. lucorum actually has discoid capitula (Jeffrey and Chen 1984;Chen 1999), so the material examined may have been misidentified or an error in observation occurred.
From the very limited chromosome data available, the most reliable basic chromosome number of Synotis appears to be x = 10, a basic number also characteristic of the genus Senecio L. as re-defined by Pelser et al. (2007). If this inference is correct, then both Synotis xinningensis and its putative closest relative, S. changiana, are tetraploid. More species of Synotis need to be examined cytologically to determine the variation pattern of the chromosomes and its systematic implications for the genus.
Synotis xinningensis is distributed in southwestern Hunan, and S. changiana in northeastern Guangxi, so their distribution ranges are adjacent to each other, only some 80 km apart (Figure 3). Both species prefer similar habitats, growing in open mixed forests at elevations between 450 and 1000 m above sea level.  Jeffrey and Chen (1984), and Chen (1999) divided Synotis into two well-marked sections, sect. Synotis and sect. Atractylidifoliae C. Jeffrey & Y. L. Chen; all but one of the species (S. atractylidifolia) fall within the former, which itself is divisible into five not very clearly differentiated series. Synotis xinningensis, with its herbaceous leaves rosulate or subrosulate at the base of inflorescence, the lower part of stem leafless at anthesis, and the inflorescence terminal, can be readily referred to ser. Synotis. In addition to S. xinningensis, series Synotis now includes 19 species (Chen et al. 2011;this study). Within series Synotis, only S. xinningensis and S. changiana have leaves that are always abaxially densely gray-white arachnoid-tomentose; all other species of series Synotis have leaves that are abaxially only sparsely arachnoid or puberulent, and often glabrescent (Jeffrey and Chen 1984;Chen 1999;Chen et al. 2011).

Conclusions
Synotis xinningensis is most similar to S. changiana, but readily distinguishable by having discoid capitula and 9-10 longer bracts of calyculus. Both species belong to Synotis ser. Synotis.