Birch pollen ‐ the key to unlock hidden cases of species hybridization

Abstract

Normal Betula pollen grains are triporate, i.e. having three pores, whereas genetically abnormal or deformed grains are usually not. We collected samples of pollen from 92 individual Betula trees/shrubs growing in natural birch woodlands in Iceland. The trees were previously identified as being diploid (2n=28) dwarf birch Betula nana (31 plants) and tetraploid (2n=56) downy birch B. pubescens (39 plants), whereas 22 plants were found to be triploid (2n=42) hybrids of the two species [1]. The results [2] showed clearly that the two species mostly produced normal triporate pollen, whereas damaged and deformed grains were significantly more frequent among pollen samples from triploid hybrids. The most frequent type of deformity in pollen morphology was pollen with four pores instead of the normal three. Meiosis in the microspore mother cells was also examined and as expected the triploid plants had irregular meiotic figures and produced deformed microspores (unpublished). We therefore investigated the fertility of these triploid trees, by testing pollen viability and assessing seed germination. The results (unpublished) confirmed that the fertility of triploid hybrids was severely reduced. The good news is that triploid hybrids are not completely sterile, and a few individuals under study are even as fertile as the parental species can be. This discovery supports our botanical and molecular studies of introgressive hybridization in Betula [1, 3], whereby triploid hybrids serve as a bridge of gene flow across the two species via back‐crossing. The knowledge that triploid birch hybrids produce abnormal pollen has been utilized in our search for past hybridization events in the Holocene vegetation history of Iceland. Samples from peat were collected in three locations in Iceland: Eyjafjördur (N), Grímsnes (SW) and Thistilfjördur (E). In all three places, periods of elevated proportions of abnormal Betula pollen were detected [4 ‐ 6]. By comparison to climate data from the Greenland Ice Core Project, the effect of climate on the birch woodlands can be seen. The hybridisation periods were found to be connected to the advance of woodland‐forming downy birch over dwarf birch habitat in warming climate. Such hybridisation may have taken place in most parts of northern Europe when woodland expanded in the beginning of the Holocene. In Iceland the climate stayed near the lower limits of birch woodland tolerance for most of the Holocene, repeatedly creating conditions that facilitated hybridization. With the warming of climate in the last few decades a new wave of birch hybridisation has started.