Multiple closely related species with genomic sequences provide an ideal system for studies on comparative and evolutionary genomics, as well as the mechanism of speciation. However, only a few genera in eukaryotes, e.g., Drosophila (Clark et al., 2007) and Oryza (Zhang et al., 2014), contain multiple genome sequenced relatives because it is expensive to sequence whole genomes at present. Fortunately, transcriptome sequencing (using RNA-seq technology) provides a rapid and effective approach to obtain large numbers of protein-coding genes, which can be used for understanding evolutionary questions in non-model organisms (Ward et al., 2012; Haas et al., 2013). Recently, comparative transcriptome studies between multiple close relatives have been used to reveal evolutionary patterns in domesticated and wild tomato (Koenig et al., 2013), and for phylogenomic analysis in Caryophyllales (Yang et al., 2015).

The genus Fragaria L. (Rosaceae) contains about 25 species, which are most spring-blooming, insect-pollinated herbaceous perennials capable of clonal growth, and have animal-dispersed fleshy fruits (Johnson et al., 2014; Liston et al., 2014). This genus has diversified mating systems and ploidy (2×, 4×, 6×, 8×, and 10×; Liston et al., 2014). Cultivated strawberry, F. × ananassa (2n = 8× = 56), is an important fruit crop, which originated via octoploid hybridization between F. virginiana and F. chiloensis in 18th century in Europe. Like natural selection, domestication has influenced the evolutionary pressures on the cultivated strawberry. The geographic distribution of Fragaria spans a wide range of Eurasia, North and South America and East Asia (Johnson et al., 2014; Liston et al., 2014). Of these, East Asia is the center of genetic diversity and the most likely original range of Fragaria (Njuguna et al., 2013). In addition, dated molecular phylogenies based on whole chloroplast genomes indicated that the genus is quite young (1.0–4.1 Mya; Njuguna et al., 2013). Obviously, this younger genus must have undergone rapid spread and species differentiation to adapt to different environments.

Previous studies have obtained the whole genome sequences of one cultivated (F. × ananassa) and five wild strawberries (F. vesca, F. iinumae, F. nipponica, F. nubicola, and F. orientalis; Shulaev et al., 2011; Hirakawa et al., 2014), which provide one of the richest genomic resources of any plant genus. Most diploid species in Fragaria have very small genomes (∼200 Mb, Table 1) compared to other higher plants. Recently evidence from linkage maps method indicated that four subgenomes of the allo-octoploid cultivated strawberry F. × ananassa were originated with diploid donors of F. vesca, F.iinumae and two F. iinumae-like ancestors (Tennessen et al., 2014; Sargent et al., 2016). A virtual diploid reference genome (173.2 Mb, contains 45,377 genes) of the cultivated strawberry was constructed by integrating the sequences of the four homoeologous subgenomes of F. × ananassa (in total 697.8 Mb, contains 230,838 genes), which eliminated both heterozygous regions as well as homeologous differences (i.e., differences between subgenomes) using overlap graph method, Hirakawa et al., 2014). Except for Fragaria, the genome sequences of other important Rosaceae fruit crops are also currently available, e.g., apple (Malus domestica), pear (Pyrus communis), peach (Prunus persica), Chinese plum (P. mume) and black raspberry (Rubus occidentalis). These abundant genome resources make it possible to improve our understanding of the genetics of horticultural traits by comparative genomics, including fruit quality, timing of flowering, and pest and disease resistance. In short, Fragaria is a very attractive model genus for the study of recent speciation, molecular ecology, evolutionary genomics and fruit development (Liston et al., 2014).

In this study, we performed RNA-seq technology to obtain most transcript sequences of seven diploid and tetraploid Fragaria species (Tables 1 and 2). Combining these with six Fragaria species genomes (including the virtual diploid genome of F. × ananassa) and seven other species which also have sequenced genomes in Rosales, we first reconstructed a phylogenetic tree based on single copy genes at the genomic level and estimated the divergence time of each node. Subsequently, the substitution rates were estimated in eight diploid strawberries to identify the rate of evolution in each species. Finally, positively selected genes (PSGs) related to agronomic characters in the cultivated strawberry were identified using evolutionary genomics. We aim to gain a comprehensive understanding of the evolutionary patterns in Fragaria at the genomic/transcriptomic level.

Our phylogenetic results based on nuclear genomes mostly agree with previous relationships determined using two nuclear genes or whole chloroplast genomes (Rousseau-Gueutin et al., 2009; Njuguna et al., 2013). The topology of major clades in the Fragaria genus is divided into three subclades with high support (100%), a Eurasian clade, an FII-FAN clade and an East Asian clade, which roughly correspond to clades A, B, and C, respectively, in the phylogenetic tree based on chloroplast genomes (Njuguna et al., 2013). East Asian species appear in all clades, indicating that this area is the center of origin of Fragaria, which is consistent with previous findings (Njuguna et al., 2013). However, there are some inconsistencies between our results and previous studies. The placement of FVI and FNG is grouped with East Asian clade with moderate support (70%) in our analysis, which has been unresolved in previous studies (Njuguna et al., 2013). In addition, FNU was incomprehensibly clustered with FOR in the Eurasian clade, which conflicts with the accepted idea that FNU is close to the East Asian clade (Rousseau-Gueutin et al., 2009). To be prudent, our natural selection analysis did not include this species’ genome sequence. Our analysis supported FAN clustering with FII rather than with Eurasian species. One reason might be that our analysis did not include the octoploid cross parents of cultivated FAN, i.e., F. virginiana and F. chiloensis, which most likely derived from diploid FVE or FMA by tetraploidization (Rousseau-Gueutin et al., 2009). Another reason is that FII is an important diploid donor of allopolyploid FAN according to resent studies, which indicated that four subgenomes of allo-octoploid cultivated strawberry derived from FVE, FII, and two with an unknown ancestor close to FII (Tennessen et al., 2014; Sargent et al., 2016). Since the virtual diploid genome of FAN was created by the overlap graph method, which is essentially a majority-rule consensus process, it should closely resemble the FII clade which provides the majority of the subgenomes. Therefore, it is understandable that our results show a close phylogenetic association between FAN and FII.

The appropriate calibration is important to the results of phylogenetic dating. In order to reduce the error, we have used two calibration constraints from two credible references (Njuguna et al., 2013; Liu et al., 2014b). From our dating results, we infer that the origin of Fragaria dates (∼7.99 Mya) from the Miocene, moderate earlier than the estimate (1.52–4.44 Mya) based on chloroplast genomes (Njuguna et al., 2013). The major reason might be that the genome sequences of the closest genus, Potentilla, were unavailable and not included in our analysis. Notably, the clade including four species (FTI, FMO, FPE, and FCO) found in the mountains of southwest China is inferred to have originated ∼3.98 Mya, which is in accordance with the time (3.6 Mya) of the strongest Qinghai–Tibet Plateau (QTP) rapid uplift in the Pliocene (Li and Fang, 1999). It suggests that the uplift of the QTP is a driving force for rapid speciation in this clade of Fragaria. That is, ecological adaptation has occurred within Fragaria following climatic and environmental changes after the QTP uplift, which promoted the speciation of these species. As has been reported for many other taxonomic groups, the QTP uplift is usually responsible for the initial isolation and subsequent climatic shifts and has driven the species divergence (Liu et al., 2012, 2014a; Zhou et al., 2012; Wen et al., 2014; Favre et al., 2015).

To investigate selective pressures in eight diploid species, we estimated substitution rates using a free model for each orthogroup. For all species, Ka/Ks ratios of most orthogroups were less than 0.5, indicating that purifying selection is the main driving force in Fragaria evolution, acting to remove deleterious mutations. The frequency distribution of Ka/Ks clearly showed that the cultivated species FAN has more genes with elevated interval values (Ka/Ks = 0.5–1.0) than the other seven species. The accelerated evolution of genes is often driven by positive selection, relaxed selection or low effective population size (Ne). During cultivation or domestication, both selection pressures for fitness traits important in the wild species and effective population size are reduced (Maceachern et al., 2009). Therefore, reduced selective constraint and reduced Ne could be responsible for the observed elevated evolutionary ratios in cultivated FAN with more elevated interval values (Ka/Ks = 0.5–1.0). While the similar number of positive selected genes (Ka/Ks > 1.0) between domesticated and wild Fragaria may be caused by artificial selection and natural selection respectively. This finding is compatible with branch-site model test for positive selection analysis, which indicated that many genes related to specific agronomic traits in FAN show significant positive selection. Moreover, the median of substitution rates (ka/ks ratio) of FAN is bigger than that of FII while smaller than FVE in free model (Figure 2A). It is noted that the substitution rates median of FAN is closer to FVE than to FII. As described above, the FAN is a virtual genome constructed from an allopolyploid via the overlap layout consensus method; we cannot rule out loci that evolved fast in FAN may actually be loci obtain from FVE rather than FII. Previous study also suggested that extensive unidirectional introgression has converted FII-like subgenomes to be more FVE-like among subgenomes (Tennessen et al., 2014). The reason might be homoploid hybridization in the FII-like diploid ancestors or strong selection spreading FVE-like sequence within the octoploid genome.

For a cultivated strawberry, many domestication-related genes in FAN involving traits desirable to humans should evolve rapidly under artificial selection, alongside natural selection caused by environmental factors. During the domestication process, fruit quality and flavor are the most important agronomic traits for strawberries. Carbohydrates, produced by photosynthesis in plants, are the basic substances of fruit growth and development and are closely related to fruit quality. In our KEGG enrichment analysis for PSGs in FAN, the “carbon fixation in photosynthetic organisms” pathway was the most significant hit. In this pathway, all four detected PSGs encode photosynthesis-related proteins (Rubisco, RPE, TPI and FBP). Rubisco is an essential and rate-limiting enzyme involved in the first step of carbon fixation and subsequently fixed into sugars (Chan et al., 1990). RPE catalyzes the reversible epimerization of D-ribulose 5-phosphate to D-xylulose 5-phosphate. TPI is involved in the gluconeogenesis and glycolysis pathways. Chloroplastic FBPase is a rate-limiting enzyme in the Calvin cycle, which plays a critical role in carbohydrate biosynthesis. A previous study reported that increasing the level of TPI and FBP aldolase in Anabaena could enhance photosynthetic activity and increase the rate of cell growth (Ma et al., 2007). We can speculate that as a trait desirable to humans, fruit quality, which can be described as the amount and composition of carbohydrates accumulated in the fruit, could undergo significant artificial selection during the domestication process. These key enzymes involved in carbon fixation and carbohydrate biosynthesis pathways show significantly positive selection further confirms this speculation.

Fruit quality and flavor are also closely associated with plant hormone biosynthesis and signal transduction (Obroucheva, 2014). Notably, the second most significantly enriched KEGG process in our results is plant hormone signal transduction, which includes eight PSGs in FAN. Among these PSGs, three homologs have similar functions in the auxin signaling process. These genes encode auxin-responsive proteins IAA11, IAA17, and IAA31, which function as repressors of early auxin response genes at low auxin concentrations (Liscum and Reed, 2002). A previous study based on an early stage fruit transcriptome in FVE revealed that IAA31 was significantly up-regulated (6.5-fold) in seeds by comparing post-fertilization and pre-fertilization stages, which suggested that auxin exerts a direct role in stimulating receptacle expansion during fruit development (Kang et al., 2013). We speculate that these PSGs involved in auxin signaling might play specific functions in early fruit development in the cultivated strawberry.

The strawberry is a non-climacteric fruit, whose ripening is directly stimulated by abscisic acid (ABA) rather than ethylene (Jia et al., 2011; Obroucheva, 2014). ABA signaling via PYR/PYL receptors begins with PP2Cs, which permit activation of SNF1-related protein kinase 2, and then regulate downstream targets such as ABA-response transcription factors. The signaling pathway of ABA involves three PSGs, including two abscisic acid-insensitive 5-like (ABI5-like) proteins, and PP2C 37 (PP2CA). Among these, PP2CA acts as a key negative regulator of ABA responses to plant growth and development, inhibition of vegetative growth, and response to glucose (Yoshida et al., 2006; Park and Cutler, 2009). For example, PP2C is highly expressed during cucumber fruit development and may be involved in regulating fruit ripening (Wang et al., 2012). Two PSGs encode ABI5-like proteins, which participate in positively regulating ABA-mediated inhibition of seed germination and growth to promote fruit maturation (Brocard et al., 2002; Chen et al., 2008; Reeves et al., 2011). Previous study has also shown that exogenous sugar can significantly promote strawberry ripening while stimulating ABA accumulation (Jia et al., 2011). Therefore, all of the above PSGs are apparently related to fruit development and ripening, which attract major attention during domestication.

TZ, JL, YZ, JH, and HS conceived and designed the project. JL, LX, QQ, TZ, YZ, HS, and QW prepared samples. QQ, TZ, QW, and LX contributed to sequencing experiment. TZ and QQ performed data analyses. TZ, QQ, and JH wrote the paper. All authors read and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.