Differentiating pollen ofBetulaspecies from Iceland

Abstract

Subfossil pollen from two co-existing Betula species in Iceland, B. nana and B. pubescens, is frequently found in sediments and peat. Interpretation of the findings often depends on the ability to differentiate between the two species according to pollen size and structure. Fresh pollen samples were prepared from 70 individual trees/shrubs which had been identified to species by chromosome number. Grain diameters and pore depths were measured and ratios of grain diameter to pore depth (D/P ratios) were calculated. The mean grain diameters of pollen from diploid B. nana and tetraploid B. pubescens were 20.42 and 24.20 mm, whereas mean pore depths were 2.20 and 2.81 mm respectively. Mean D/P ratios were therefore 9.55 for B. nana and 8.85 for B. pubescens. The difference between species was statistically significant for all three pollen parameters. Grain diameter appeared to be the most useful parameter, as only about 20% of the samples were in the overlapping region of the species distributions. Pollen size (grain diameter) was also positively correlated to tree morphology, which was evaluated using species-specific botanical characters. Pollen samples from different locations/ populations in Iceland varied slightly in mean size and ratio. The size difference between pollen of B. nana and B. pubescens in this study is less than other papers have reported, which may be due to the effect of introgressive hybridisation between the two birch species in Iceland.

Characteristics of pollen from natural triploidBetulahybrids

“…All samples were prepared under the same laboratory conditions. The analysis of pollen from the diploid B. nana and the tetraploid B. pubescens was reported in Karlsdó ttir et al (2007), and as a continuation of that work the analysis of pollen from the triploid plants is presented here in comparison with pollen of the two parental species.…”

“…The pollen samples were prepared and mounted in silicon oil as in Karlsdó ttir et al (2007). Images of 120 -130 pollen grains from each individual tree/ shrub were taken using a Nikon digital camera DXM1200F on a Nikon Eclipse E800 microscope with 4006 microscopic magnification.…”

“…But the chromosome number of Icelandic birch plants is stable: only three groups of plants have been identified -diploid, triploid and tetraploid plants (Thó rsson et al, 2001. Pollen of diploid B. nana was therefore compared with that of tetraploid B. pubescens; the results showed significant difference in the mean size of pollen grains of these two species, regardless of the large overlap in plant morphology (Karlsdó ttir et al, 2007).…”

“…No clear characteristics of pollen separate species within the genus Betula, but differences in mean size, relative size of vestibulum and minor structural features of pores make identification on species level just about possible (Blackmore et al, 2003). In our recent study (Karlsdó ttir et al, 2007), pollen size was measured and compared between samples of B. nana and B. pubescens that were classified based on chromosome number. Due to the introgressive hybridisation described above, morphological variation in Icelandic Betula is so great that species identification based on morphology is rendered difficult.…”

Combined LM and SEM study of the middle Miocene (Sarmatian) palynoflora from the Lavanttal Basin, Austria: part IV. Magnoliophyta 2 – Fagales to Rosales

“…. of many extant Betula species has been presented by Birks (1968), Lieux (1980), Zavada and Dilcher (1986), Chen (1991), Jones et al (1995), Wang et al (1995), Mäkelä (1996), Fritz and Allesch (1999), Blackmore et al (2003), Wei (2003), Beug (2004), Clegg et al (2005), Karlsdóttir et al (2007Karlsdóttir et al ( , 2008, Li et al (2011a), Miyoshi et al (2011) and Lin et al (2013). The pollen ultrastructure (SEM, transmission electron microscopy [TEM]) of Betula has been described and figured by Zavada and Dilcher (1986), Pehlivan (1987) and Blackmore et al (2003).…”