alleles, one being homozygous and the other heterozygous on chromosome IV (Tennessen et al., 2013; Ashman et al., 2015). In F. orientalis and F. moschata, Staudt (1967a,b) found tetrasomic inheritance for sex and he described the alleles for sex as male suppressor SuM (F) dominant to male inducer Su+ (H) and to the female suppressor SuF (M). SuF was dominant to Su+.
The sexual determining regions (SDR) of the octoploids have now been sequenced and mapped in four geographically distinct octoploid taxa: F. virginiana ssp. virginiana, F. virginiana ssp. platypetala, Fragaria cascadensis and F. chiloensis (Spigler et al., 2008, 2010, 2011; Goldberg et al., 2010; Tennessen et al., 2018). The SDR cassette contains two putatively functional sex-determining genes that moved at least three times by likely transposition, as evidenced by the size of their flanking sequence, which increased with each ‘jump’ (Tennessen et al., 2018). The SDR of F. virginiana ssp. virginiana, F. virginiana ssp. platypetala and F. chiloensis are found on a unique section of a chromosome from the same homoeologous group, but from a different subgenome. F. cascadensis male function maps to the same subgenome as in F. virginiana spp. platypetala but at a different chromosomal position (Wei et al., 2017b).
Tennessen et al. (2013, 2014) used targeted-sequence capture to map the sex determination regions in F. vesca ssp. bracteata. They identified locations affecting sexual phenotype on two chromosome regions, one on LG4 and another on LG6. The dominant allele (R) on LG6 appears to restore fertility in the absence of a male sterile allele at the LG4 locus. The F. vesca locus contains a high density of pentatricopeptide repeat genes, a class commonly involved in restoration of fertility caused by cytoplasmic male sterility. They also found evidence of a third unmapped locus influencing sex phenotype. The gene on LG6 in F. vesca is on the same chromosome as the one regulating sex in F. chiloensis but at a different position. However, these two genes are likely not homologous, as the one in F. chiloensis is dominant while the one in F. vesca is recessive.
DESCRIPTION AND LOCATION OF STRAWBERRY SPECIES
Diploids (2n = 2x = 14)
Fragaria bucharica Losinsk.
This species is similar to Fragaria nubicola except it has sympodial rather than monopodial runners (Staudt, 2006; Hummer et al., 2011). Two subspecies are recognized based on the size of their bractlets: F. bucharica ssp. bucharica(larger) and F. bucharica ssp. darvasica (smaller). Fragaria nubicola can be crossed with F. mandshurica, F. vesca and F. viridis, resulting in mostly heterotic plants with the morphological characters of F. bucharica. Crosses with Fragaria nipponica produce dwarf plants. F. bucharica is distributed from Tadjikistan to Afghanistan, Pakistan and Himachal Pradesh in India.
Fragaria chinensis Losinsk.
This species is a slender plant about 8–15 cm in height with monopodial branching runners (Lei et al., 2014). Its leaves are trifoliate, elliptic or obovate and nearly sessile. Its runners and peduncles are glabrous or covered with sparse appressed hairs. There are two to six flowers per inflorescence with a calyx that is wide lanceolate to triangular. Its fruits are pale red to red and mostly flavourless, with light yellow to brown seeds that are deeply sunken in the fruit. F. chinensis is native to western and south-western China.
Fragaria vesca L.
The wood or alpine strawberry is cultivated to a limited extent in North America and Europe. It has thin, light-green, sharply serrated leaves borne on slender petioles (Fig. 1.3). The branching of its stolons is sympodial. The terminal tooth of the terminal leaflet is usually longer than the adjacent lateral teeth and the calyx is reflexed. The plant is erect and 15–30 cm tall. Flowers are bisexual, approximately 1.3 cm wide; inflorescences are about the same length or taller than the leaf petioles. Most plants are short day, but everbearing types exist (F. vesca f. semperflorens). Fruits are long ovate, bright red in colour and highly aromatic. The fruit has very soft flesh and raised or superficial seeds. Runnerless and white-fruited forms exist.
Fig. 1.3. Duchesne’s drawing of Fragaria vesca. Cytogenetic studies suggest that this species may be a diploid progenitor of the octoploid strawberries. (From Darrow, 1966.)
There are four subspecies found in the group (Staudt, 1962, 1999): (i) F. vesca ssp. vesca – woods of Europe and Asia; (ii) ssp. americana (Porter) Staudt – woods of eastern North America to British Columbia; (iii) ssp. bracteata (Heller) Staudt – woods of western North America; and (iv) ssp. californica (Chamisse and Schlechtendal) Staudt – California. Several ecotypes have been described within ssp. californica including headland scrub, coastal forest and Sierran forest (Table 1.2). All of these subspecies are hermaphroditic and self-fertile, except for ssp. braceata which has both hermaphrodites and occasional females (Staudt, 1989).
Table 1.2. Ecotypes of F. vesca and F. chiloensis found in California. (From Hancock and Bringhurst, 1979a, b.)
Fragaria viridis Duch.
This is a slender, upright species with dark-green leaves with smaller serrations than F. vesca (Fig. 1.4). It is native to Europe and central Asia, and found in open grassland hills, steppes, at the edge of forests and among brush. It produces only a few nodeless runners with monopodial branching. Flower numbers per inflorescence are smaller than F. vesca, but it has perfect flowers that are larger than F. vesca. The petals overlap and are often yellowish-green when opening. Fruit is small but larger than F. vesca, firm, green to pink in colour, and aromatic. The scapes lie along the ground when the berries are ripe. Seeds are set in pits. The calyx is clasping and hard to separate. F. viridis can be distinguished from F. vesca by its phosphoglucose isomerase isozyme pattern (Arulsekar and Bringhurst, 1981).
