Variations in Genome Size of Turnip Landraces from Two High-altitude Environments

Brassica rapa var. rapa (turnip) is considered a main source of food for the inhabitants of the QinghaiTibetan Plateau (QTP) and its adjacent highlands when other crops are scarce. The QTP ranges from lat. 25.598N to 39.498N and from long. 73.298E to 104.408E, whereas the Yunnan Plateau ranges from lat. 20.008N to 29.168N and from long. 96.008E to 110.198E. A comparison between the turnip landraces of two different plateau environments can provide a mechanistic insight into plant adaptation in highlands. The aim of this investigation was to understand the patterns in variation in genome size (GS) between the turnip landraces of two plateau environments. We used a wellestablished protocol to count chromosome number and performed propidium iodide flow cytometry to measure GS. No polyploidy was detected among the turnip landraces tested, and 15.5% variation in GS was observed between the landraces. No consistent pattern pertaining to GS variation emerged after the environmental variables were considered. Thus, we propose that such pattern may reflect the indirect effect of selection, random process, genetic drift, or some other factors on GS through interaction of life-form and phenotypic traits. One of the goals of ecology is to explain the patterns of species abundance across different regions. Such patterns are assessed by investigating the phenotypic traits of congeneric taxa, such as seed size/number, plant height, and specific leaf area. Genome size (GS) is the amount ofDNA in an unreplicated, basic, and gametic chromosome set; the regional abundance of a species can be predicted by using GS as a proxy for plant phenotypic traits because GS covaries with the rate of cell division and cell size (Knight and Beaulieu, 2008). Hence, GS is proposed as a key determinant constraining plant fitness over ecological time (Knight et al., 2005). For example, researchers who studied large-scale cytotype distribution (McAllister et al., 2015) hypothesized that higher GS plants adapt better in high altitudes than lower GS plants. However, the results of some studies are inconsistent with this hypothesis (Guo et al., 2016). For instance, Bennett et al. (1982) indicated that lower GS plants have increased tolerance in highlands. These contrasting ideas illustrate that higher GSmay be a necessary but not critical to life in high-elevation areas (Knight and Ackerly, 2002). However, this hypothesis remains to be validated experimentally for flowering plants adapted to cultivation in the Qinghai-Tibetan Plateau (QTP) and the Yunnan Plateau (YP), which may strengthen knowledge in plant domestication in the highlands. QTP is one of the world’s largest biodiversity hotspots. It has an average altitude of more than 4500 m and is thus called ‘‘the roof of the world. ’’ QTP had undergone a dramatic change in climate during the past million years, such as conversion of forests to grasslands; formation of glaciers and desserts; arid and chilly climate (Wu et al., 2001). Meanwhile, YP, situated at an average elevation of 2000 m, is known as a hotspot for plant diversity (Ruth et al., 2008). In QTP, environmental extremes have led to the evolution of many flowering plants at the intraspatial scale, whereas, flora in YP exhibits particularly strong spatial and ecological isolation because of its unique landscape and climate. Intraspatial comparative phylogenomic study between the two areas shows two distinct populations resulting from the fragmentation of two plateaus by periods of glaciation and subsequent interruption in gene flow (Li et al., 2011). Moreover, species adapted to QTP show higher genetic diversity than in YP because of their enhanced tolerance to extreme climate (Li et al., 2011). Low temperature, high ultraviolet radiation, and drought severely affect the physiology of highland vegetation and shape GS by imposing selection pressure on the physiology and life history traits correlated to GS (Knight and Ackerly, 2002). Unfortunately, intraspatial GS variation among landraces that adapted to cultivation inQTP andYP is rarely investigated. In this study, we investigated GS variation between the landraces grown in QTP and YP at the intraspatial scale. In flowering plants, intraspatial GS research has considerable gaps. Although there are several studies reporting the existence of intraspatial genome plasticity, these results were either doubted or rejected on taxonomical and technological grounds. Feulgen densitometry was used in the late 19th century to compare GS variation; however, this method was not appropriate to assess intraspecies GS variation. This has changed because flow cytometry became available, as it provides the necessary accuracy to obtain estimates of intraspecies GS variation. But usage of internal standards, consideration of cytosolic inhibitors, and documentation of double peak by cochopping plant specimen are absolutely Received for publication 26 Jan. 2018. Accepted for publication 8 Mar. 2018. We thank Yuanwen Duan for statistical advice. This work was financially supported by Yunnan Postdoctoral Grants to S. Basak, Major State Basic Research Development Program (2010CB951704), and National Natural Science Foundation of China (NSFC) (41271058) to Y. P. Yang. Corresponding author. E-mail: yangyp@mail.kib. ac.cn. 136 J. AMER. SOC. HORT. SCI. 143(2):136–143. 2018. important for flow cytometry to provide evidence for intraspecies GS variation. Hence, there was necessity to generate standard operating procedures for identifying intraspecies variation of GS throughout the world. During the last decade, several groups developed standardized protocols for measuring intraspecies GS variation (Dole zel and Barto s, 2005). Recent investigation based on flow cytometry confirmed that true intraspatial variation exists among angiosperms, but the level of intraspatial variation that can be accommodated by a species remains unclear (Benor et al., 2011). Moreover, whether GS variation is governed by a selection process is unknown so far. Although sequencing is the most powerful technique for the investigation of adaptive evolution in angiosperms, GS calculated from sequencing data is still prone to errors because of the misassembly of repetitive genomes. Despite the availability of GS estimates for 220 species of 4060 members from Brassicaceae family (5%) (Bennett and Leitch, 2011), intraspatial GS information remains scarce (but see Long et al., 2013). The mechanism by which GS has been shaped by ecological and evolutionary forces among cultivated members of this family is unknown. Turnip is an important biennial, outcrossing, mesopolyploid herbaceous plant in the Brassicaceae family and is native throughout Europe, Russia, Central Asia, and Near East. Europe has been proposed to be the center of origin. In the alpine regions of QTP, turnip was widely used as a food source when other crops are scarce. The highlands of QTP which range in altitude between 3000 and 4500 m are characterized by strong ultraviolet-B radiation and low accessibility of water (Liu et al., 2012). Turnip is able to germinate during cold and drought. It also grows at high elevation (up to 1000 m in Kumaon Himalaya, India) in tropical countries, such as India, Nepal, Bhutan, Bangladesh, Sri Lanka, and Australia. Because of its AA genome, turnip is one of the ancestor species of Brassica juncea (2n = 36, genome AABB) and Table 1. Information of the collection sites and distinguishing morphological characteristics of 23 Brassica rapa var. rapa landraces. From this table, it is evident that Yunnan Plateau (YP) landraces selected in this investigation are relatively from lower latitude depicting the geographical distribution of YP compared with Qinghai-Tibetan Plateau (QTP). Moreover, morphological description of leaf length, aboveground biomass, tuber diameter, and underground biomass for 23 landraces from the two plateau environments demonstrate that YP landraces are bigger than QTP landraces.