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Discussion
Identification of Flowering Genes in Strawberry
Genetic regulation of flowering in strawberry has earlier been studied only by
crossing experiments. According to Weebadde et al. [61], everbearing character is
a polygenic trait in garden strawberry whereas other studies indicate the presence
of a single dominant gene [62]. Different results may arise from different origin
of everbearing habit, since at least three different sources have been used in straw-
berry breeding [32,61,62]. Studies in F. vesca 'Baron Solemacher'have shown that
EB flowering habit in this genotype is controlled by recessive alleles of a single
locus, called seasonal flowering locus (sfl) [40,41]. Identification of central genes
regulating flowering, as well as those controlling other processes that affect flower-
ing (runnering, chilling), is an important goal that would greatly accelerate breed-
ing of strawberry and other soft fruit and fruit species of Rosaceae family.
For comprehensive identification of candidate genes of the strawberry flower-
ing pathways, we searched homologs for 118 Arabidopsis flowering time genes
from our own cDNA libraries and from GDR. In total, we were able to identify
66 gene homologs among about 53000 EST sequences. Moreover, gene homologs
lacking from Fragaria were further mined from Rosaceae EST collections contain-
ing about 410 000 EST sequences. These searches revealed 22 additional putative
flowering time genes in Rosaceae. Ongoing genome sequencing projects in apple,
peach and wild strawberry will ultimately reveal the currently lacking flowering
regulators in these species [63].
Sequences found in Fragaria corresponded to all known Arabidopsis flower-
ing time pathways [2] suggesting that all of these genetic pathways may be pres-
ent in Fragaria. However, the sequence conservation does not necessarily mean
functional conservation, so major candidate genes from different pathways have
to be functionally characterized in order to prove the presence of these pathways
in strawberry. Few central regulators of flowering time are lacking from Fragaria
sequence collections and some of them also from Rosaceae databases. For ex-
ample, we were not able to identify a homolog for the florigen gene FT [11] in
Fragaria regardless of several different attempts. This is probably due to its low
expression level and tissue specific expression pattern [64]. Similarly, GI, which
links circadian clock and CO [8,65], was absent from the Fragaria sequences. FT
and GI homologs were, however, found in apple and Prunus, showing that they
are present in Rosaceae. Moreover, consistent with studies in model legumes [66],
CCA1 was lacking in Rosaceae, but its redundant paralog, LHY, is represented by
few ESTs in Fragaria. CCA1 and LHY are MYB-type transcription factors which
repress the expression of TOC1 in the central loop of Arabidopsis circadian clock
[67]. Thus, in Fragaria and other species of Rosaceae family, LHY alone may
