Horticultural Reviews, Volume 49. Группа авторов

the first genetic map has recently been developed in H. macrophylla (Waki et al. 2018). Selecting for traits which cannot be reliably phenotyped until maturity, with markers, will increase breeding efficiency by allowing the breeder to know what characteristics to expect early in the breeding cycle, thereby potentially reducing the breeding cycle by several years. Markers linked to double flower and inflorescence type genes in H. macrophylla are a prime examples (Waki et al. 2018; Wu et al., 2020).

      Microsatellites have been developed for Hydrangea that are informative at both the inter‐ and intraspecific level (Rinehart et al. 2006; Reed and Rinehart 2007, 2009; Choi et al. 2017; Hempel et al. 2018; Waki et al. 2018). Microsatellites are sequences of several nucleotides that repeat several times consecutively (Gupta et al. 1996). Thirty‐nine SSRs have been developed for H. macrophylla (Reed and Rinehart 2007), 14 of which were informative for determining species relationships within the genus, including H. quercifolia (Rinehart et al. 2006). Microsatellites have also been utilized in H. luteovenosa to estimate the genetic diversity and conservation status of a critically endangered population in South Korea (Ito et al. 2013; Choi et al. 2017). Recently, a genetic map of H. macrophylla with 147 SSR loci was developed (Waki et al. 2018); it is unknown if these markers would be useful in H. quercifolia.

      Single nucleotide polymorphisms (SNPs) are highly abundant mutations throughout genomes (Semagn et al. 2006). SNPs are either single base‐pair substitutions or insertion/deletion polymorphisms and are therefore considered to be biallelic markers. Genotyping by sequencing (GBS) is a technique which harnesses high‐throughput sequencing technology to simultaneously identify and genotype thousands of SNPs by reducing genomic complexity, using restriction enzymes prior to sequencing (Elshire et al. 2011). GBS has recently been used inH. macrophylla (Tränkner et al. 2019; Wu and Alexander 2019) andH. quercifolia (Sherwood et al. 2020).

I. Genetic Variation in Hydrangea

      Genetic variation is an important metric for plant breeding and conservation alike. For plant breeders, genetic variation is the raw material to be utilized in order to make gains from selection. In conservation, genetic variability is likewise extremely important, as a species that possesses higher genetic variability has an increased ability to adapt to changing environmental conditions and selective pressures.

Natural variation has been increasingly utilized for breeding and genetic studies (Xue et al. 2008; Alonso‐Blanco et al. 2009; Weigel 2012). A major benefit of using natural variation in studying plant genetics or plant improvement is that natural intraspecific variation can exceed (or greatly differ from) that found in cultivated genepools. For example, initial horticultural characterizations of wild collected oakleaf hydrangea seedlings revealed variation in the species exceeding that which is found among cultivars for traits such as leafspot tolerance and resistance to deacclimation (Sherwood et al. 2021), and further characterization is expected to reveal further novel variation for additional traits. This can be applied to hydrangea breeding by selectively incorporating wild germplasm into a breeding program in addition to existing cultivars.

      Although genetic variation between species of Hydrangea and within H. macrophylla, H. paniculata, and H. febrifuga (Rinehart et al. 2006, 2010; Reed and Rinehart 2007, 2009) has been determined, no comprehensive studies have previously been undertaken for H. quercifolia. Additionally, none of these studies examined the full range of variation in the species, but rather focused on the available germplasm in the form of cultivars and limited numbers of wild accessions. Studies of species‐wide genetic diversity and population structure in H. quercifolia are currently underway, and suggest that considerable diversity exists in the species, with unique genetics existing in different parts of the species range (Sherwood et al. 2020) which also correspond to unique phenotypic diversity (manuscript in preparation).

      A recent high‐resolution study of population structure of a disjunct and geographically isolated island population of H. luteovenosa revealed extremely low levels of genetic diversity in the population (Choi et al. 2017). Using five SSR markers, it was determined that only two distinct genotypes exist on Jeju Island, South Korea, one of which dominates the small population (251 out of 285 samples) likely due to clonal reproduction. This indicates that an extremely limited extent of diversity exists in the three Japanese populations of the species occurring on the disjunct Jeju Island. The fact that one of the two genotypes is found in 88% of the population indicates that the population has an extremely limited ability to adapt to changing conditions, and as such is at a higher risk of extirpation and is more susceptible to inbreeding than the Japanese populations.

      Population genetic structure and phylogeography of H. platyarguta (syn. Platycrater arguta) has also been studied across the extent of its natural range in eastern China and southern Japan (Qiu et al. 2009). Using chloroplast DNA sequences, this study found two genetically and geographically distinct groups which correspond to var. sinensis and var. arguta. Overall, it was shown that H. platyarguta has high genetic variation.

VIII. CONSERVATION

Although H. quercifolia is abundant in the center of its range in central and northern Alabama, there is nevertheless a conservation concern. Many of the existing populations on the extremities of the range are small and, therefore, are susceptible to genetic drift and inbreeding. This could also be an early indication of range contraction and local extirpation. Additionally, the extent of gene flow between populations as either seed or pollen is unknown, which has a direct influence on the extent of genetic drift in small populations. Knowledge of these factors will be useful in assessing the need for conservation action as well as directing any action that is deemed necessary.

      A survey of 421 herbarium specimens of H. quercifolia indicates that the average collection year was 1980, with only 16% having been collected since 2000. Searching for populations throughout the native range indicates that approximately 23% (16 of 69 searched) of the populations that were previously documented no longer exist (A. Sherwood, pers. observ.). Many of these extirpated populations are located on the edges of the species range, and therefore put an emphasis on protecting the populations in those areas. These locations have either undergone land‐use change (housing developments, clear cutting, roadside vegetation control, etc.) or encountered habitat degradation due to invasive species, indicating that land preservation could be an effective strategy in protecting the remaining populations. Molecular marker data suggest that these edge populations in the southern extent of the range are genetically distinct and unique (Sherwood et al. 2020), further supporting the need for directed management strategies in the areas where H. quercifolia is not abundant.

IX. PROPAGATION

      Hydrangea quercifolia is readily propagated from seed and vegetative cuttings. Seeds are extremely small (0.6 mm long; Hufford 1995) but require no stratification treatment in order to achieve germination. Surface sown seeds typically germinate in 7–14 days with or without a light covering, so long as seeds are not allowed to dry out or become buried under germination substrate (Halcomb and Reed 2012). Overall, germination rates are fairly low, on average 62% (Sherwood et al. 2019); however, this is within the range observed for H. paniculata (37–65%) and H. macrophylla (5–65%; Greer and Rinehart 2009). Additionally, seed germination significantly varies by mother plant and population in wild collected seed (Sherwood et al. 2019). Seed germination percentages can also be increased by germinating in a growth chamber rather than a greenhouse (Sherwood et al. 2021). Oakleaf hydrangea roots readily from softwood cuttings, although IBA may increase the rate of rooting (Halcomb and Reed 2012).

H. quercifolia can be propagated via tissue culture. When introducing explants into culture, microbial contamination is a widespread problem in Hydrangea (Kitamura et al. 2008; A. Sherwood, unpubl.). However, the use of dormant buds as explant material has been used successfully in H. quercifolia (Sebastian and Heuser 1987; Ledbetter and Preece 2004). Both type and concentration of cytokinin were shown to have significant effects on number and length of shoots induced in tissue culture, with 5 μM thiadiazuron