Morphological Variation among Betula nana (diploid), B. pubescens (tetraploid) and their Triploid Hybrids in Iceland

Abstract

Hybridization between B. nana and B. pubescens is widespread in Iceland. The species can be distinguished from each other morphologically, and from the triploid hybrids. The overlapping morphological variation indicates bidirectional introgression between the two species via triploid hybrids. Iceland could be considered a birch hybrid zone, harbouring genetic variation which may be advantageous in subarctic regions.

Characteristics of pollen from natural triploidBetulahybrids

“…As defined in the introduction, we do not separate B. pubescens in Iceland into subspecies, which is based on morphology. Repeated investigations (recently published in Thó rsson et al, 2007) have shown that the hybrid-like morphology (tortuosa-type) is found in all ploidy groups of Icelandic woodland birch samples, and most interestingly only half of the triploid birch has hybrid morphology. In the study reported here, we examined pollen from triploid birch plants, which was formed under meiotic restrictions and only a very small fraction of microspores survived to be normal and viable pollen grains (unpublished results).…”

“…All other numbers must have been eliminated, either via gametic or sporophytic selection, as only backcrossed progeny having diploid, triploid and tetraploid chromosome numbers were recovered. No aneuploid was found among these backcrossed progeny, nor has any aneuploid ever been found in Icelandic woodlands (Anamthawat-Jó nsson & Thó rsson, 2003;Thó rsson et al, 2007). Unreduced gametes (3 6) might have been formed among the hybrid pollen, but no 5 6 or 6 6 birch plants have ever been discovered.…”

“…Pollen samples were collected from 92 individual trees/shrubs in ten different sites in Iceland: Brekkuskó gur, Bifrö st, Á sbyrgi, Eidar, Jö kulsá í Ló ni, Skaftafell, Reykjanes, Kjálkafjö rdur, Kaldalón and Mývatn (for description of the sites and map, see Thó rsson et al, 2007). All of these plants had been identified beforehand by chromosome number, which was obtained from direct counting of metaphase chromosomes from leaf bud cells as in Anamthawat-Jó nsson (2003b).…”

Divergent levels of genetic variation and ploidy among populations of the rare shrub, Grevillea repens (Proteaceae)

“…Two possible explanations for the formation of the triploids are cross pollination between a diploid plant (2x) and an extant (or historical) tetraploid (4x) or the union of a haploid (x) and an unreduced (2x) gamete (Husband 2004). In some taxa, partially fertile triploid plants can act as a bridge for gene flow between diploid and tetraploid species (Thórsson et al 2007) or intraspecific cytotypes (Husband 2004). It is not known if the triploid G. repens cytotypes are sterile but it is likely that fertility in these plants is strongly reduced and at least partial barriers to reproduction exist between the two ploidy variants.…”

Calculated characters of leaves are independent on environmental conditions in Salix herbacea (Salicaceae) and Betula nana (Betulaceae)

“…Studies on Betula nana focused on its occurrence in the early Holocene floras [29][30][31], hybridization with Betula pubescens [28,[32][33][34][35][36] and importance in forming the mountain birch (Betula pubescens var. tortuosa) [32][33][34][35][36][37][38]. In Scandinavia, B. nana builds vast populations and shows low morphological variation.…”