CULTIVATED FLAX HAS BEEN UNDER DISRUPTIVE SELECTION'

Flax is an unusual crop plant due to double purpose of its cultivation. On one hand, it is grown for its oil-rich seed, on the other - for its fibrous stems. Oil content is

positively correlated with seed size (Diederichsen & Raney 2006) while fibre content is correlated with stem height and branching pattern (Diederichsen & Ulrich 2009). The correlation analysis carried out in this project for over a thousand cultivated flax samples shows that seed size and plant height are negatively correlated (Section

5.3.1, Figure 5.2) that supports the expectation that both agronomic traits cannot be

selected for at the same time. Plants with smaller seeds were selected for fibre production, while shorter, big-seeded plants were favoured for oil production. A similar observation was made by Kulpa and Danert (1962) when they classified flax cultivars into specialised convarieties. Furthermore, this line of evidence is congruent with the output of simulations carried out in the programme PGROWTH (Section 5.3.3, Figure 5.7). Fibre and oil yield improvement require the opposite resource allocation during plant development, in vegetative or generative growth respectively. Hence, it is likely that the two varieties have been under disruptive selection.

One way of identifying disruptive selection in flax is to investigate its population structure based on molecular data. Strong structure could support the presence of such selection. RADseq data used in this project allowed the discernment of four and six subpopulations in INSTRUCT and STRUCTURE analyses, respectively (Section 6.3.2, Figure 6.3). However, none of these were consistent with the division between fibre and oil convarieties of flax. Contrary to the expectations, landraces, intermediate, fibre and oil varieties had a similar genetic make-up. The difference was only

observed after sorting the samples within varieties according to the latitude. It turned out that landraces, intermediate and fibre varieties are characterized by strong

division between northern and southern samples. Oil varieties on the other hand, do not have such structure. Even though the separation of oil and fibre varieties is not possible based on the RADseq molecular markers in this study, an important difference was observed: fibre flax was subject to a disruptive selection in different latitudes while oil flax was not.

The signature of disruptive selection for southern- and northern-adapted varieties of flax might reflect the impact of adaptive haplotypes on genetic isolation. The

LuTFL1 haplotype data. There is a correlation between haplotype variants and the latitude (Section 4.3.2, Figure 4.1b). The genetic network of LuTFL1 haplotypes marks a clear separation between southern cluster I and northern cluster III (Section

4.3.2, Figure 4.1a). Finally, it was shown that the distribution of haplotypes in

LuTFL1 network could be explained by the presence of selection or a selective sweep (Section 4.3.3, Table 4.3). In light of this evidence, it becomes clear that latitude might have had a huge impact on genetic separation of northern flax populations not only through isolation by distance but by the means of selection. This notion further supports the presence of disruptive selection in cultivated flax. However, this selection is likely to be driven by the need to adapt to northerly latitudes through change in flowering habit.

a 109 87 103 99 100 94

b

c III I III III II VIII

d

Figure 7.2: Flowering time in six accessions of pale flax and their genetic background.

a – Time from sowing to the occurrence of first floral buds in days. b – Strength of expression of LuFT on 91st and 110th day after sowing. c – LuTFL1 haplotype.

d – Assignation to genetic populations as measured by STRUCTURE.

There might be other genetic factors that alter flax flowering strategies. The genetic network of flowering in model plants is very complex (Flowers et al. 2009; Hall et al. 2011) and there are likely many genes regulating this process in flax. Flowering time measured in six wild accessions of pale flax was highly variable even between two

Turkish accessions. Hence, some diversity in this phonological trait existed prior to adaptation to Central European climate. Flowering time diversity is independent from variation within LuTFL1 gene since it is not correlated with its haplotypes (Figure 7.2). The number of tested samples is far to small to make any statistically significant inferences, however, these results are congruent with expression pattern of LuFT gene. Additionally, late flowering accession W043 is genetically separated based on STRUCTURE analyses and is the only accession from the direct

neighbourhood of the domestication centre. It might be the case that some other genetic factor played a role in changing cultivated flax flowering time through regulation of LuFT gene even before it left Turkey.

LuTFL1 might have conferred flax adaptation to Central European climate through change in mode of growth (from determinate to indeterminate) rather than change in flowering time. Change from determinate to indeterminate growth was crucial in the adaptation of soybean to Northern China (Tian et al, 2010). A remaining question pertinent to this study is whether the flax adaptation to Central Europe was linked with the emergence of fibre varieties.

7.5THEORETICAL INFERENCE ABOUT THE EMERGENCE OF FLAX FIBRE VARIETIES