Abstract
Recent molecular and karyologic studies have significantly modified delimitation of Lilium. However, despite the importance of pollen evolution in the genus comprehensive studies with electron microscopy and evaluation of pollen evolution are lacking. Therefore, we studied pollen morphology in a sample of 65 individuals from 37 taxa covering all the sections distributed in the world, using scanning electron microscopy. Our collection of 49 individuals from 21 taxa covering all five sections in China was also included in the database. We found pollen tetrads in L. bakerianum. Based on present and previous studies, our results suggest that pollen from L. formosanum should be classified as a new type, Formosanum. Combined with morphological and molecular evidence, pollen sculpture patterns appear to reflect phylogenetic relationships and are useful for species or subsection delimitation. Based on a comprehensive survey and correlation with potential functional implications, we propose the following hypothesis: evolution of an exine sculpture shows pollen type trends from Martagon → Callose → Concolor → Formosanum. The evolutionary trend regarding pollen sculpture and size could be related to selective pressure to adapt to environmental conditions. Pollen size and shape showed a significantly positive correlation with annual precipitation, and smaller pollen grains appear to adapt better in habitats with extreme conditions. Evolution trends in exine sculpture do not appear to be definitively correlated with pollen size and shape.
Highlights
110 to 115 Lilium species are distributed in the cold and temperate regions of the Northern Hemisphere [1,2], in East Asia, the Himalayas and Hengduan Mountains, North America and Europe
By combining previous and present results, our aim is to provide palynology information regarding the extent of pollen morphological diversity within and between species/sections, link this information with current phylogenetic data, and contribute to a comprehensive understanding of the systematics and evolution of Lilium by: (1) verifying pollen types in Chinese species; (2) discussing these findings within a phylogenetic framework; and (3) correlating pollen morphology and size with habitat
According to the Lilium classification by Comber [8], Nishikawa et al [18,19] and Du et al (2013) [16], palynological results for each section are as follows: Martagon Representatives of Martagon (Figure 1A–F) have pollen size ranging from L. distichum Nakai et Kamibayashi (91.56634.57 mm, P/E of 2.66) to L. tsingtauense Gilg (104.70644.52 mm, P/E of 2.36)
Summary
110 to 115 Lilium species are distributed in the cold and temperate regions of the Northern Hemisphere [1,2], in East Asia, the Himalayas and Hengduan Mountains, North America and Europe. De Jong [3] and Patterson and Givnish [4] consider southwest China and the Himalayas to be the center of origin of this genus. Classification of this genus has been historically complicated. Lophophorum (Bur. et Franch.) Wang et Tang out of sect. Lophophorum to accommodate the Nomocharis-like Lilium species in sect.
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