Náttúrufræðingurinn - 2016, Page 26

Náttúrufræðingurinn
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tant species in Icelandic vegetation.
Woodland birch in Iceland is low-grow-
ing and often polycormic, probably as a
consequence of hybridisation between
the two species. The species have differ-
ent chromosome numbers and therefore
their hybrids produce mostly abnormal
gametes. But a few healthy ones are suf-
ficient to backcross to the parental spe-
cies, allowing gene flow between them,
the process known as introgressive hy-
bridisation or introgression. Abnormal
Betula pollen grains in peat or sedi-
ments thus indicate past hybridisation.
In this study we searched for such pol-
len in order to find hybridisation peri-
ods during the Holocene. Samples from
peat were collected in three locations in
Iceland: Grímsnes, Eyjafjörður and
Þistilfjörður. Betula pollen grains were
measured and the species proportions
calculated from size. Abnormal Betula
pollen grains were counted separately.
In all three places, periods of elevated
proportions of abnormal Betula pollen
were detected. By comparison to climate
data from the Greenland Ice Core
Project, the effect of climate on birch
woodlands could be seen. The hybridi-
sation periods were found to be con-
nected to the advance of woodland-
forming downy birch over dwarf birch
habitat under 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 condi-
tions that facilitated hybridisation. With
the warming of climate in the last few
decades a new wave of birch hybridisa-
tion has started.
Heimildir
1. Elkington, T.T. 1968. Introgressive hybridization between Betula nana L.
and B. pubescens Ehrh. in North-West Iceland. New Phytologist 67. 109–118.
2. Kesara Anamthawat-Jónsson og Ægir Þór Þórsson 2003. Natural hybrid-
isation in birch: Triploid hybrids between Betula nana and B. pubescens.
Plant Cell, Tissue and Organ Culture 75. 99–107.
3. Ægir Þór Þórsson, Snæbjörn Pálsson, Lascoux, M. og Kesara Anamthawat-
Jónsson 2010. Introgression and phylogeography of Betula nana (diploid),
B. pubescens (tetraploid) and their triploid hybrids in Iceland inferred from
cpDNA haplotype variation. Journal of Biogeography 37. 2098–2110.
4. Ramsey, J. og Schemske, D.W. 1998. Pathways, mechanisms, and rates of
polyploid formation in flowering plants. Annual Review of Ecology and
Systematics 29. 467–501.
5. Caseldine, C. 2001. Changes in Betula in the Holocene record from Ice-
land – a palaeoclimatic record or evidence for early Holocene hybridisa-
tion? Review of Palaeobotany and Palynology 117. 139–152.
6. Lilja Karlsdóttir, Margrét Hallsdóttir, Ægir Þór Þórsson og Kesara
Anamthawat-Jónsson 2009. Frjókorn fjalldrapa og ilmbjarkar á Íslandi.
Náttúrufræðingurinn 77. 107–112.
7. Lilja Karlsdóttir, Margrét Hallsdóttir, Ægir Þór Þórsson og Kesara
Anamthawat-Jónsson 2009. Evidence of hybridisation between Betula
pubescens and B. nana in Iceland during the early Holocene. Review of
Palaeobotany and Palynology 156. 350–357.
8. Lilja Karlsdóttir, Margrét Hallsdóttir, Ægir Þór Þórsson og Kesara
Anamthawat-Jónsson 2012. Early Holocene hybridisation between Betula
pubescens and B. nana in relation to birch vegetation in Southwest Iceland.
Review of Palaeobotany and Palynology 181. 1–10.
9. Lilja Karlsdóttir, Margrét Hallsdóttir, Ólafur Eggertsson, Ægir Þór Þórs-
son og Kesara Anamthawat-Jónsson 2014. Birch hybridization in Thistil-
fjördur, North-east Iceland during the Holocene. Icelandic Agricultural
Sciences 27. 95–109.
10. Lilja Karlsdóttir 2014. Hybridisation of Icelandic birch in the Holocene
reflected in pollen. Doktorsritgerð við Líf- og umhverfisvísindadeild
Háskóla Íslands.
11. Bhattacharya, C.G. 1967. A simple method of resolution of a distribution
into Gaussian components. Biometrics 23. 115–135.
12. Pauly, D. og Caddy, J.F. 1985. A modification of Bhattacharya’s method
for the analysis of mixtures of normal distributions. FAO Fisheries Circu-
lar 781. FAO, Róm. 16 bls.
13. Prentice, I.C. 1981. Quantitative birch (Betula L.) pollen separation by
analysis of size frequency data. New Phytologist 89. 145–157.
14. Morgan, J.H. 2005. A computer method for resolving mixed normal dis-
tributions. Ringing and Migration 22. 145–152.
15. Margrét Hallsdóttir og Caseldine, C.J. 2005. The Holocene vegetation
history of Iceland, state-of-the-art and future research. Bls. 319–332 í:
Iceland: Modern Processes and Past Environments (ritstj. C.J. Caseldine,
A. Russell, Jórunn Harðardóttir og O.F. Knudsen). Elsevier, Amsterdam.
16. Rasmussen, S.O., Vinther, B.M., Clausen, H.B. og Andersen, K.K. 2007.
Early Holocene climate 10 oscillations recorded in three Greenland ice
cores. Quaternary Science Reviews 26. 1907–1914.
17. Vinther, B.M., Clausen, H.B., Sigfús Jóhann Johnsen, Rasmussen, S.O.,
Andersen, K.K., Buchardt, S.L., Dahl-Jensen, D., Seierstad, I.K., Siggaard-
Andersen, M.L., Steffensen, J.P., Svensson, A.M., Olsen J. og Heinemeier,
J. 2006. A synchronized dating of three Greenland ice cores throughout
the Holocene. Journal of Geophysical Research 111, D13102. DOI:
10.1029/2005JD006921
18. Grace, J., Berninger, F. og Nagy, L. 2002. Impacts of climate change on the
tree line. Annals of Botany 90. 537–544.
19. Reitsma, T. 1969. Size modification of recent pollen grains under differ-
ent treatments. Review of Palaeobotany and Palynology, 9. 175–202.
20. de Groot, W.J., Thomas, P.A. og Wein, R.W. 1997. Betula nana L. and Bet-
ula glandulosa Michx. Journal of Ecology 85. 241–264.
21. Kesara Anamthawat-Jónsson og Þorsteinn Tómasson 1999. High frequency
of triploid birch hybrid by Betula nana seed parent. Hereditas 130. 191–193.
22. Wöll, C. 2008. Treeline of mountain birch (Betula pubescens Ehrh.) in Ice-
land and its relationship to temperature. Diploma thesis in Forest Botany.
Technical University Dresden. 134 bls.
23. Vasari, Y. og Vasari, A. 1990. L’histoire holocène des lacs islandais. Bls.
277–293 í: Devers, S. (ritstj.) Pour Jean Malaurie: 102 témoignagés en
hommage á quarante ans d’études arctiques. París, Plon. 944 bls.
24. Margrét Hallsdóttir 1995. On the pre-settlement history of Icelandic
vegetation. Búvísindi 9. 17–29.
25. Wang, N., Borrell, J.S., Bodles, W.J., Kuttapitiya, A., Nichols, R.A. og
Buggs, R.J. 2014. Molecular footprints of the Holocene retreat of dwarf
birch in Britain. Molecular Ecology 23. 2771–2782.
26. Eidesen, P.B., Alsos, I.G. og Brochmann, C. 2015. Comparative analyses
of plastid and AFLP data suggest different colonization history and
asymmetric hybridization between Betula pubescens and B. nana. Molecu-
lar Ecology 24. 3993–4009.
27. Ellstrand, N.C. og Schierenbeck, K.A. 2006. Hybridization as a stimulus
for the evolution of invasiveness in plants? Euphytica 148. 35–46.
28. Rieseberg, L.H., Kim, S.C., Randell, R.A., Whitney, K.D., Gross, B.L.,
Lexer C. og Clay, K. 2007. Hybridization and the colonization of novel
habitats by annual sunflowers. Genetica 129. 149–165.
29. Christensen, J.H., Hewitson, B., Busuioc, A., Chen, A., Gao, X., Held, I.,
Jones, R., Kolli, R.K., Kwon, W.-T., Laprise, R., Magaña Rueda, V.,
Mearns, L., Menendez, C.G., Räisänen, J., Rinke, A., Sarr, A. og Whetton,
P. 2007. Regional climate projections. Bls. 847–940 í: Solomon, S., Qin, D,.
Manning, M., Chen, Z., Marquis, M., Averyt, K.B, Tignor, M. og Miller,
H.L. (ritstj.). Climate Change 2007: The Physical Science Basis. Contribu-
tion of Working Group I to the Fourth Assessment Report of the Inter-
governmental Panel on Climate Change. Camridge, Engl. og New York,
Bandar., Cambridge University Press.
30. Halldór Björnsson, Árný E. Sveinbjörnsdóttir, Anna K. Daníelsdóttir, Árni
Snorrason, Bjarni D. Sigurðsson, Einar Sveinbjörnsson, Gísli Viggósson,
Jóhann Sigurjónsson, Snorri Baldursson, Sólveig Þorvaldsdóttir og
Trausti Jónsson 2008. Hnattrænar loftslagsbreytingar og áhrif þeirra á
Íslandi – Skýrsla vísindanefndar um loftslagsbreytingar. Reykjavík,
Umhverfisráðuneytið. 120 bls.
31. Truong, C., Palmé, A.E. og Felber, F. 2007. Recent invasion of the mountain
birch Betula pubescens ssp. tortuosa above the treeline due to climate
change: Genetic and ecological study in northern Sweden. Journal of
evolutionary biology 20. 369–380.