Hybrid introgression: the outcomes of gene flow in birch

Kesara Anamthawat-Jónsson

ABSTRACT: Understanding life history of natural forest and woodland is important as we need to prepare for, to cope with and to tackle problems associated with the ongoing global environmental changes. The present paper focuses on one of the world?s terrestrial biomes, the arctic tundra, which is a type of biome where the tree growth is hindered by low temperatures and short growing seasons. A severe threat to tundra is global warming, which causes permafrost to melt affecting species survival, and which releases soil-bound carbon in the form of greenhouse gases. Tundra vegetation is composed of dwarf shrubs, sedges and grasses, mosses and lichens - a woodland type of vegetation dominated primarily by birch shrubs. The aim of this paper is to summarize and highlight my thirty three years of genetic studies of birch woodlands in Iceland, from the discovery of introgressive hybridization in birch and estimating the extent of introgression, to uncovering past hybridization events since the end of the last glaciation about 9000 years ago. The studies use different approaches, from cytogenetic and botanical, to molecular and palynological. The overall results show clearly that hybridization between B. nana and B. pubescens is widespread in Iceland; the resulting gene flow via introgressive hybridization is bi-directional; and that the process is continuous through time and space. Iceland could be considered a birch hybrid zone, harbouring genetic variation which may be advantageous in subarctic regions. However, waves of hybridization associated with climate warming could drive one of the species into extinction.

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^a	Institute of Life and Environmental Sciences, University of Iceland, Sturlugata 7, IS-101 Reykjavik, Iceland

* Corresponding author, E-mail: kesara@hi.is

Received 28 Jun 2019, Accepted 30 Jun 2019