Genetic variability of grey poplar hybrids (Populus x canescens =

P. alba x P. tremula) and the significance of hybrids

for adaptation

Berthold HEINZE

Federal Research Centre for Forests, Department of Genetics, Vienna,

AustriaTBX Partner No. 2

Natural poplar hybrids

• often occur where „compatible“ parent species overlap

• show a high diversity of characters– Mendelian segragation beyond F1

• but the parent species do not „collapse“– they stay distinct species

• could natural hybrids contribute to adaptation?

Our starting point: phylogeography of 2 hybridizing poplar species

in Central Europe• P. tremula and P. alba• contrasting habitats

– upland – boreal vs. riverine

• hybridizing freely where they meet– backcrosses and introgression

• natural clones may persist• our hypothesis: individual genes have a

different tendency for introgression – making functional analysis possible

Populus alba Populus tremula

Populus x canescens

flowering phenolgy of both species overlaps in Vienna

Populus alba: female flowers and fruits Populus tremula: female and male flowers

• depending on weather conditions in each year

• in-situ and ex-situ observations

Phylogeography:genetic maps showing

phylogenetic relationships

• different scales of geographic areas require different genetic markers

• chloroplast DNA molecule often useful for identifying range-wide groupings

• evolves slowly (many copies in a plant)• clonal evolution (no recombination)• DNA sequence changes in length

(insertions/deletions) or restriction sites detectable and easily analysed

A „simple“ genome:

the Populus chloroplast

functions of ~ 15% of genes still unknown

some highly variable regions

Technique: PCR-RFLP-electrophoresis

• universal primers– database at: http://bfw.ac.at/rz/bfwcms.web?

dok=977– preliminary results from years of experience

• restriction analysis

• agarose gels

• fragment analysis and sequencing on a capillary electrophoresis automat for one fragment

• stepwise variation of 16bp

• P. alba und P. tremula

• visible on agarose gels, but analysed on sequencer

Tandem repeat structure in chloroplast gene rpl16 intron

tandem repeats in Populus rpl16 intron

In Europe, chloroplast lineages often tell something about

range-shifts during the glaciations

• tree populations restricted to southern penninsulas during ice maxima

• isolated evolution

• secondary contact after ice retreated

• today‘s genetic patterns contain information on possible re-colonization routes

P. tremula and P. alba chloroplast variants

in Central Europe

• high number of variants (haplotypes), but a few main ones dominate

• two groups of haplotypes separate the species– but hybridization and introgression „moves“ some

variants into hybrids and into the other species• Italian and Balkan Penninsulas as main sources for P.

alba• no clear structure in P. tremula

High diversity of chloroplast types

typical P. tremula

typical P. alba

... but hybrids can have haplotypesfrom either side (arrows) ...

High diversity in Central Europe

Two Populus species show different patterns

of geographical differentiation in Central and South-Eastern Europe

• practically no chloroplast DNA differentiation in P. tremula

• P. alba shows patterns of re-immigration

• different glacial refugia and secondary contact after / during re-immigration

P. tremula types are little differentiated

P. alba types are quite differentiated

STRUCTURE suggests clear species differentiation,

and some population differentiation

• K=24 order by Q

K=2

K=24

is the genetic shape of hybrids a product of these

phylogeographic differences only, or do cyto-nuclear interactions

interfere?

• flowering phenology suggest that P. alba can capture chloroplasts from P. tremula more easily

• are all chromosomes equally affected by cyto-nuclear disequilibrium?

* * * * * * * * * * * * * * * * * * * * * * * * * * *

Most P x canescens are F2 (NewHybrids)

• extensive cyto-nuclear interactions• as the cyto-nuclear interactions do not affect all

chromosomes equally, migration and drift alone

are insufficient reasons for this phenomenon

Postglacial admixture of P. tremula lineages

in Scandinavia

• P. tremula in Scandinavia – de Carvalho/Lexer/Ingvarsson, manuscript in

press Molec. Ecol.– using 70+ nuclear microsatellites

Christian Lexer and Pär Ingvasson

Postglacial admixture of P. tremula lineages in Scandinavia

Christian Lexer and Pär Ingvasson

Clinal variation, but ...

• Increased variance for bud set in the centre of the cline

• Stronger selection in the centre of the cline

• the cline is rather a step than continuous

• great variation in marker ancestry along e.g. chromosome 6, known to exhibit normal levels of recombination– Yin et al. 2004 TAG

• Low variation in marker ancestry along chromosome 19, consistent with reduced recombination– Yin et al. 2008 Genome Res.

Conclusions – P. tremula Scandinavia

• Adaptive population divergence dectable at the European scale, indicating that recombination is lower or selection greater than previously thought

• Postglacial contact zone of P. tremula in N-Europe coincides with those postulated for many other species

• Admixture facilitates adaptation from standing variation, as visible from cline shapes, variances, and selection differentials

• Great deal of variation for marker ancestry across the genome = great potential of admixture mapping

General conclusions

• Admixture – most often of post-glacial lineages in Europe – contributes to the adaptation potential of tree species

• Admixture of hybridizing species may involve subtle interactions– e.g. cyto-nuclear ones

• Populus sp. are an excellent model to study admixture and adaptation

Acknowledgements• grants by Austria Academy of Science –

Jubiläumsfonds der Stadt Wien, DOC fFORTE (to Barbara Fussi)

• sampling support: – Cvrčková H., Máchová P. (FGMRI, CZ), Bartha D. (Sopron,

HU) Benke A. (ERTI Sarvar, HU), Bogdan S. (Zagreb, HR), Gonzales-Martinez S. (Madrid, E), Nica M.S., Ipati A., ICAS (RO), and Castiglione S. (Salerno, IT)

• lab support: Bianca Widmar, Renate Slunksy, summer students (B. Sokcevic, K. Duran)

• thank you for your kind attention!