Primulina serrulata (Gesneriaceae), a new species from southeastern Guizhou, China

Abstract Primulina serrulata R.B.Zhang & F. Wen, a new species from a limestone area in southeastern Guizhou, China, is described and illustrated here. The new species is morphologically related to P. fimbrisepala (Hand.-Mazz.) Y.Z.Wang. We examined the morphological differences between these congeners and provide illustrations and photographs of this new species in this paper.


Introduction
The formerly monotypic genus, Primulina Hance, has become the largest genus in the subfamily Didymocarpoideae of Gesneriaceae in China (Weber et al. 2013, Xu et al. 2017, Wen et al. 2019). The number of Primulina has been growing explosively since 2005 (Möller et al. 2016, Möller 2019. For the moment, there are a total of 199 species (including infraspecies) in this genus in China (Wen et al. 2019). Primulina shows high levels of endemism and ecological (edaphic) specialization, especially in Karst areas.
Based on extensive literature surveys and field observations, we discovered that most species of Primulina occur in the karst area of southern and southwestern China and northern Vietnam. They are often limited to a single or a few caves or in specialized and narrow microhabitats of karst limestone hill systems, called island distributions (Wang et al. 1998, Li and Wang 2004, Wei et al. 2010, Hao et al. 2015. The current research result indicated that geographical isolation had been demonstrated to be one kind of reliable driver of Primulina diversification and speciation, for example, P. eburnea and P. hochiensis complexes (Gao et al. 2015, Wang et al. 2017, Yang et al. 2019. In 2018, during field explorations, a local herbalist found an unknown species of Primulina near the Guizhou-Guangxi border at Rongjiang County, southeastern Guizhou, China (Figure 1). Several living individuals from the population found in the field were brought to the nursery of Gesneriad Conservation Center of China (GCCC) and cultivated there. The leaf blade margin characteristics of these plants look appealing because of the prominent and irregular serrations. At the same time, the beautiful silvery veins on the leaf blades and purplish-blue flowers soon caught our attention. However, this species can easily be distinguished from its congener by its morphology. After a morphological comparison between this new species and its related species and literature studies, we consider it is new to science, which is being described and illustrated here.

Methods
Morphological observations of the new species were carried out based on living plants as well as dry specimens. All morphological characters were measured with dissecting microscopes and were described using the terminology presented by Wang et al. (1998). Literature studies included all relevant monographs, i.e., Wang et al. (1998), Li and Wang (2004) and Wei et al. (2010), and also some recently published literature. Specimen checking was done at IBK, IBSC, PE and also some web databases, such as Chinese Virtual Herbarium (http://www.cvh.ac.cn/) and Global Plants (http://plants.jstor.org/).
Distribution and habitat. At this time, Primulina serrulata is only known from the type locality in Langdong village, Langdong town, Rongjiang County, Guizhou Province, based on our field investigations. It grows on moist, shady, limestone rocks near a waterfall, at ca. 780 m altitude, with no more than 150 mature individuals. The population is close to a road, which makes it vulnerable and subject to destruction from human activities.
Phenology. This new species was observed flowering in April and fruiting from May to June.
Etymology. The specific epithet is derived from its particular leaf blade margin, having obvious serrations and bi-serrations.
Provisional IUCN conservation assessment. Because of Primulina serrulata's beautiful leaves and flowers, it is being over-harvested by local people for sale. This unpublished species is therefore on the brink of extinction as a result. Before more surveys are completed to clarify its conservation status, the provisional conservation status is Critically Endangered CR B2ab (iii, v) according to the IUCN red list criteria (IUCN 2012).
Note. Primulina serrulata is related to its congener, P. fimbrisepala, by some characteristics, for example, the similar calyx lobes and infundibuliform corolla tube, but they can easily be distinguished from each other by the characters summarized in the description below. Numerous dark purple spots were covered at the throat of the corolla, and this is one stably distinctive feature of P. fimbrisepala. It is noticeable in different populations of P. fimbrisepala from South China when they are flowering (Figure 4). Table 1 below has more detailed information on how to distinguish the two species. They also grow in different substrates. P. serrulata only grows in limestone areas, while P. fimbrisepala commonly appears in weakly acidic mediums and soils of sandstone or granite mountainous regions. The different growing habitats of the two congeners indicate that geographical isolation should be one of the reliable drivers pushing this genus, Primulina, to generate diversification and speciation (Gao et al. 2015, Wang et al. 2017, Yang et al. 2019.