Abstract
Genetic diversity was estimated for 51 Lagerstroemia indica L. cultivars, five Lagerstroemia fauriei Koehne cultivars, and 37 interspecific hybrids using 78 simple sequence repeat (SSR) markers. SSR loci were highly variable among the cultivars, detecting an average of 6.6 alleles (amplicons) per locus. Each locus detected 13.6 genotypes on average. Cluster analysis identified three main groups that consisted of individual cultivars from L. indica, L. fauriei, and their interspecific hybrids. However, only 18.1% of the overall variation was the result of differences between these groups, which may be attributable to pedigree-based breeding strategies that use current cultivars as parents for future selections. Clustering within each group generally reflected breeding pedigrees but was not supported by bootstrap replicates. Low statistical support was likely the result of low genetic diversity estimates, which indicated that only 25.5% of the total allele size variation was attributable to differences between the species L. indica and L. fauriei. Most allele size variation, or 74.5%, was common to L. indica and L. fauriei. Thus, introgression of other Lagestroemia species such as Lagestroemia limii Merr. (L. chekiangensis Cheng), Lagestroemia speciosa (L.) Pers., and Lagestroemia subcostata Koehne may significantly expand crapemyrtle breeding programs. This study verified relationships between existing cultivars and identified potentially untapped sources of germplasm.
There are more than 50 species of Lagerstroemia L. (Cabrera, 2004; Furtado and Montien, 1969), but L. indica and L. fauriei have been the most extensively used in horticultural breeding programs. Lagerstroemia indica, native to southeast Asia, is a medium to large multistemmed shrub with pink to brown bark and simple, glabrous, deciduous leaves that change from green to yellow, orange, or red in the fall. The flowers of L. indica range from 2 to 5 cm long. Lagerstroemia indica cultivars perform well as ornamental shrubs or trees in U.S. Department of Agriculture cold-hardiness Zones 7 through 9.
Lagerstroemia fauriei is found only on the Japanese island of Yakushima, has strong resistance to powdery mildew (Erysiphe lagerstroemia E. West), is slightly more cold-hardy than L. indica, and has an appealing exfoliating bark. Flowers are small, white, and blooming occurs only once per season (Creech, 1985). Lagerstroemia fauriei was introduced into the U.S. National Arboretum breeding program in 1956 through seeds collected in Japan. When crossed, L. indica and L. faurei produce various desirable combinations of ornamental traits such as interesting growth habits and bark colors; a wide range of flower colors, including white, pink, purple, and red; and resistance to powdery mildew and some insect pests. Over 200 crapemyrtle cultivars exist (Dix, 1999) with at least half of these available from wholesale and retail nurseries. There are 32 crapemyrtle cultivars that are protected by U.S. patents.
Most cultivars selected before the latter part of the 20th century were L. indica seedlings chosen for unique flower color or growth habit (Egolf and Andrick, 1978). Subsequent crapemyrtle breeding has been primarily limited to interspecific hybridizations between L. indica and L. fauriei. Wild-collected L. indica germplasm from China is not readily available. Additional unique L. fauriei germplasm may not exist because it is a rare species, possibly a single population, localized to one island in Japan. Therefore, most crapemyrtle breeding programs seek new genetic combinations by crossing existing cultivars to create new combinations of ornamental traits. Genetic diversity estimates are critical for crapemyrtle breeding, germplasm management, and conservation strategies because of inbreeding depression (Pounders et al., 2006).
Breeding for new crapemyrtles also incorporates pest resistance found in existing cultivars. Many cultivars with L. fauriei in their pedigree show resistance to two common fungal diseases that affect crapemyrtles, powdery mildew and leaf spot (Cercospora lythracearum Heald & Wolf) (Hagan et al., 1998; Williams et al., 1998). Lagerstroemia fauriei cultivars also appear to exhibit differential resistance to flea beetles [Altica spp. Geoffroy (Coleoptera: Chrysomelidae)] and japanese beetle [Popillia japonica Newman (Coleoptera: Scarabaeidae)], two of the most common insect pest of crapemyrtles (Cabrera et al., 2008; Pettis et al., 2004). Several L. indica cultivars appear to have some resistance to the crapemyrtle aphid [Tinocallis kahawaluokalani Kirkaldy (Hemiptera: Aphididae)] (Herbert et al., 2009).
In recent years L. subcostata, L. limii, and L. speciosa have also been used in crapemyrtle breeding (Dix, 1999; Pounders et al., 2007a). Lagerstroemia subcostata has large flowers but displays a more limited range of flower colors (lavender, pink, white) and growth habits than L. indica. Lagerstroemia speciosa is a large tree species that exhibits desirable flowering performance and display. However, desirable traits from both of these tropical species must be introgressed into cold-hardy backgrounds because their usefulness as ornamental plants is limited to southern Florida, coastal California, and Hawaii. Conversely, L. limii has sufficient cold-hardiness and disease resistance to be grown in temperate regions, but it has small flowers and lacks ornamental appeal.
Interspecific hybrids between L. indica and L. fauriei show no apparent loss of fertility (Pounders et al., 2006). Chromosome number is not reported for L. fauriei, but cross-compatibility with L. indica suggests that it is consistent with the basic chromosome number of x = 8 for family Lythraceae J. St.-Hil. nomen conservandum (Raven, 1975; Tobe et al., 1986). Bowden (1945) and Guha (1972) report 2n = 50 for L. indica, whereas Ali (1977) reports 2n = 48. Repeated interspecific hybridizations among L. indica, L fauriei, L. limii, and L. subcostata indicate broad compatibility among species (Pooler, 2006a, 2006b). However, hybridizations between L. indica ×fauriei ‘Tonto’ and L. speciosa only produced sterile progeny suggesting cytogenetic differentiation may interfere with some combinations (Pounders et al., 2007a). Chromosome number for L. speciosa is reported as 2n = 50 by Bowden (1945) and 2n = 48 by Guha (1972).
The first genetic diversity studies on Lagerstroemia were conducted by Pooler (2003), in which the diversity of 12 L. fauriei clones was revealed by amplified fragment length polymorphism (AFLP) and random amplified polymorphism DNA (RAPD) molecular markers. The objectives of this study were 1) to compare pedigree information and parentage of selected cultivars released by the U.S. National Arboretum and other breeders using molecular diversity data; 2) to assess the genetic diversity between L. indica and L. fauriei; and 3) to evaluate shared allele sizes using pedigree-based analyses. Alleles associated with important horticultural traits could be used to accelerate future crapemyrtle breeding through molecular marker-assisted selection (MAS).
SSRs were used to DNA fingerprint 93 crapemyrtle cultivars because they are ubiquitous for most eukaryotic genomes, are codominant, and provide useful assessments of genetic diversity (Goldstein and Pollock, 1997; Pollock et al., 1998; Rossetto, 2001; Wang et al., 1994). SSRs have been used to confirm inter- and intraspecific hybridization, verify parentage, and assign genetic distinctions among cultivars of woody ornamental landscape crops (Caetano-Anollés et al., 1999; Pounders et al., 2007a; Rinehart et al., 2006). Some of the SSR loci used in this study are expected to cross-amplify in other Lagestroemia species for future research on species diversity (Pounders et al., 2007a).
Materials and Methods
Plant material.
Fifty-one L. indica cultivars, five L. fauriei cultivars, and 37 interspecific hybrid cultivars were used in this study (Table 1). Lagerstroemia limii and L. subscostata were included as outgroups for phenetic analyses. To guard against plant mislabeling, leaf samples were collected from multiple sources for most cultivars. Duplicate samples from multiple sources produced 100% identical genotype data, as expected for vegetatively propagated plants, and only one sample was analyzed further. Uneven sample sizes and deviations from Hardy-Weinberg equilibrium (HWE) resulting from selection did not alter phenetic comparisons, which are based on genetic distance estimates.
Characteristics of 93 Lagerstroemia cultivars used this study, including growth habit and flower color, which are the two most important horticultural traits for consumers.
Sample processing.
SSR development has been described by Wang et al. (2010) and the PCR protocol is the same as was used in Rinehart et al. (2006) and Waldbieser et al. (2003). Briefly, SSR-enriched libraries were made from genomic DNA of L. indica ‘Whit IV’ and L. indica ×fauriei ‘Tonto’ using the SSR motifs GA, AAG, ATG, and CAG. From 684 potential SSR loci that were identified, 96 polymorphic loci were tested on all samples (Table 2). DNA was extracted from 1 × 1-cm sections of fresh leaf tissue using a Plant Mini Kit (Qiagen, Valencia, CA) and polymerase chain reaction (PCR) amplification was performed using a three-primer protocol in 96-well plates using a thermocycler (Tetrad; Bio-Rad Laboratories, Waltham, MA). Fluorescence-labeled PCR fragments were visualized by automated capillary gel electrophoresis on an ABI3130xl using ROX-500 size standard (Applied Biosystems, Foster City, CA). GeneMapper Version 4.0 was used to recognize and size peaks (Applied Biosystems).
Description of the simple sequence repeat loci used in the analysis of 93 Lagerstroemia cultivars and interspecific hybrids, including repeat motifs, primer sequences, and statistical analyses of results.
Data analysis.
Data from 96 SSR loci were compiled for 93 samples and analyzed. Allele frequencies, mean number of alleles per locus (A), and allelic richness (Rs) (El Mousadik and Petit, 1996) were computed using FSTAT 2.9.3.2 software (Goudet, 2001). Gene diversity (Nei, 1973) estimates were produced using Nei's (1987) estimator for shared allele frequencies using Genepop 4.0.10 (Raymond and Rousset, 1995). Estimates of heterozygote deficit [Wright's fixation index (Fis)] (Wright, 1978) overall loci was obtained using FSTAT software. Significance of Fis was determined using the randomization test implemented in FSTAT (Weir and Cockerham, 1984).
Cluster analysis.
Populations 1.2.30 was used for phenetic analyses (Langella, 2002). Genetic distances between individual samples were calculated using shared allele distance to create a matrix (Jin and Chakraborty, 1993). Unweighted pair group method with arithmetic mean (UPGMA) with 100 bootstrap replicates was used to generate a dendrogram showing clustering of genetically similar samples (Saitou and Nei, 1987; Takezaki and Nei, 1996). Two species, L. limii and L. subcostata, were included as outgroups for rooting the dendrogram. Dendrograms were visualized with TreeView 1.6.6 (Page, 1996).
Results
All SSR loci used were trinucleotide repeats and amplified alleles (amplicons) were all in the expected size range. Of the 96 SSR loci examined, 18 loci were excluded from analyses to avoid possible distortion of genetic diversity estimates. Of these, four loci exhibited only one allele size (monomorphic) across all samples but were not known to be polymorphic in other species. Four loci had a high (greater than 10%) frequency of missing data within the 93 samples even after repeated amplification. The remaining 78 SSR loci were polymorphic, showed high reproducibility, had a low frequency of missing data (less than 5%), and were used for diversity study of the selected Lagerstroemia cultivars.
Gene diversity within and between groups (L. indica, L. fauriei, and interspecific hybrids), number of alleles, genotypes detected by per pocus, allelic richness per locus, and heterozygosity deficit were calculated overall loci for the 93 cultivars examined (Table 2). Allelic richness (Rs) for all samples ranged from 3.6 (locus 517_518) to 11.8 (locus 593_594) with an average of 6.4 alleles detected per locus. SSR loci detected a total of 1061 genotypes (ranging from four to 36 per locus) with an average of 13.6 genotypes detected per locus. Measures of genetic diversity overall samples varied considerably between loci. Observed genetic diversity (Ho) was calculated for each locus and ranged from 0.043 (locus 275_276) to 0.993 (locus 361_362) with a mean of 0.491. The total genetic diversity (Ht = 0.665) was slightly higher than the value observed (Ho = 0.491), indicating a higher heterozygote deficit (Fis = 0.174) (Table 2).
Total gene diversity (Ht) was calculated for each locus; the mean for all loci was 0.665, whereas the average gene diversity for each locus was 0.541 (Table 2). If samples are randomly chosen from within a group (L. indica, L. fauriei, or interspecific hybrids), they should differ on average at 54.1% of their loci. If they are chosen from the whole sample, the differences increase to 66.5%. Genetic differentiation (Gst) and average gene diversity (Dst) values between species were 0.181 and 0.126, respectively. Thus, only 18.1% of the overall variation was the result of differences between groups (L. indica, L. fauriei, and interspecific hybrids), whereas diversity among groups was 12.6%. Locus 593_594 was the most informative with the highest gene diversity (Ht = 0.861) and detected the most unique genotypes (G = 36) and number of alleles (A = 12). Locus 361_362 showed the highest observed heterozygosity overall samples (Ho = 0.993), whereas locus 275_276 showed the least heterozygosity (Ho = 0.043) (Table 2).
Genetic similarities among individuals sampled are shown in the dendrogram in Figure 1, which was rooted with L. limii and L. subscotata. Clustering among cultivars and hybrids is in general agreement with their genetic background and pedigrees (Fig. 1; Table 1). As expected, three main clusters can be inferred from this dendrogram. The L. indica cluster includes all L. indica cultivars except Red River, which is labeled as L. indica but located in the interspecific hybrid cluster. The interspecific hybrid cluster includes 28 of 37 selected interspecific hybrids between L. indica and L. fauriei. The other nine interspecific hybrids clustered with L. indica. All five L. fauriei cultivars examined were tightly grouped in a single cluster (Fig. 1).
Dendrogram of 93 Lagerstroemia cultivars generated by unweighted pair group method with arithmetic mean (UPGMA) cluster analysis using shared allele distance (DAS). The tree is rooted with L. limii and L. subcostata. Three main clusters represent cultivars derived from L. indica, L. fauriei, and interspecific hybrids between the two species. Asterisks indicate nine cultivars that cluster with L. indica but supposedly contain other Lagerstroemia species in their genetic background (see Table 1). Bootstrap values greater than 50 were limited to relationships between two and three samples to the right of the dashed line and are not shown.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 136, 2; 10.21273/JASHS.136.2.116
Dendrogram of 93 Lagerstroemia cultivars generated by unweighted pair group method with arithmetic mean (UPGMA) cluster analysis using shared allele distance (DAS). The tree is rooted with L. limii and L. subcostata. Three main clusters represent cultivars derived from L. indica, L. fauriei, and interspecific hybrids between the two species. Asterisks indicate nine cultivars that cluster with L. indica but supposedly contain other Lagerstroemia species in their genetic background (see Table 1). Bootstrap values greater than 50 were limited to relationships between two and three samples to the right of the dashed line and are not shown.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 136, 2; 10.21273/JASHS.136.2.116
Dendrogram of 93 Lagerstroemia cultivars generated by unweighted pair group method with arithmetic mean (UPGMA) cluster analysis using shared allele distance (DAS). The tree is rooted with L. limii and L. subcostata. Three main clusters represent cultivars derived from L. indica, L. fauriei, and interspecific hybrids between the two species. Asterisks indicate nine cultivars that cluster with L. indica but supposedly contain other Lagerstroemia species in their genetic background (see Table 1). Bootstrap values greater than 50 were limited to relationships between two and three samples to the right of the dashed line and are not shown.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 136, 2; 10.21273/JASHS.136.2.116
Discussion
This is the first large-scale evaluation of genetic relationships among crapemyrtle cultivars and is expected to be useful in efficiently combining existing genetic diversity in novel ways. Significant deviations from HWE were detected for all loci (data not shown) and are associated with positive Fis values, revealing deviation in the direction of heterozygote deficiency, but this is expected for samples derived exclusively from vegetatively propagated cultivated materials. Gene diversity measures were also calculated for pure species L. indica and L. fauriei overall loci and were 0.578, 0.367, and 0.656 for L. indica, L. fauriei, and interspecific hybrids, respectively. When analyzing pure L. indica and L. fauriei samples only, Dst and Gst values are 0.169 and 0.255, respectively, slightly higher than those when including interspecific hybrids. Thus, 25.5% of the overall variation is the result of differences between these species, whereas diversity among species was 16.9%, indicating that a large number of the allele sizes is shared between these species. These results support the observed fertility after interspecific hybridization between L. fauriei and L. indica and the large number of cultivars derived from hybrid parents. Further estimates of species diversity are needed to build a roadmap for efficient and successful interspecific hybridizations to capitalize on untapped genetic potential.
In general, clustering based on UPGMA grouped cultivars and hybrids in agreement with their pedigree. As expected, the L. fauriei cluster contains all five L. fauriei samples. Pooler (2003) produced three distinct groups among 12 clones of L. fauriei using AFLP and RAPD markers. Although only five L. fauriei cultivars were included in this analysis, clustering agrees with the groups reported in Pooler (2003). The L. indica group contains 50 of the 51 L. indica cultivars. The missing cultivar, Red River, was purchased from a commercial nursery and may have been mislabeled. Mislabeling can occur in crapemyrtle production when plants that are similar in flower color and growth habit are propagated, transported, and sold. Alternatively, ‘Red River’ lacks a robust description and may be a hybrid although it is labeled as L. indica.
The hybrid cluster contains 28 of the 37 interspecific hybrids between L. fauriei and L. indica as well as hybrids containing L. limii such as ‘Arapaho’ and ‘Cheyenne’. As noted in Table 1 and depicted in Figure 2, most hybrid crapemyrtles are derived from complex crosses containing varying percentages of L. indica and L. fauriei. For example, ‘Arapaho’ and ‘Cheyenne’ contain L. limii but are primarily composed of L. indica and L. fauriei so their inclusion in the hybrid group is not unexpected.
Familial relationships between Lagerstroemia cultivars released by the U.S. National Arboretum are depicted by pedigree with cultivar names listed below symbols. Circles indicate plants used as maternal sources and squares for paternal contributions. Filled symbols indicate L. indica, and open symbols indicate L. fauriei. Gray symbols indicate hybrid plants between L. indica and L. fauriei, whereas gradients were used to mark other Lagerstroemia species contributions. ‘Apalachee’ could not be connected to other pedigrees and is shown separately in B. ‘Low Flame’, ‘Red’, and ‘Basham's Party Pink’ are shown more than once because they were used as maternal and paternal parents of several cultivars. Cultivar names marked with asterisks were not included in this study; descriptions of all other cultivars can be found in Table 1.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 136, 2; 10.21273/JASHS.136.2.116
Familial relationships between Lagerstroemia cultivars released by the U.S. National Arboretum are depicted by pedigree with cultivar names listed below symbols. Circles indicate plants used as maternal sources and squares for paternal contributions. Filled symbols indicate L. indica, and open symbols indicate L. fauriei. Gray symbols indicate hybrid plants between L. indica and L. fauriei, whereas gradients were used to mark other Lagerstroemia species contributions. ‘Apalachee’ could not be connected to other pedigrees and is shown separately in B. ‘Low Flame’, ‘Red’, and ‘Basham's Party Pink’ are shown more than once because they were used as maternal and paternal parents of several cultivars. Cultivar names marked with asterisks were not included in this study; descriptions of all other cultivars can be found in Table 1.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 136, 2; 10.21273/JASHS.136.2.116
Familial relationships between Lagerstroemia cultivars released by the U.S. National Arboretum are depicted by pedigree with cultivar names listed below symbols. Circles indicate plants used as maternal sources and squares for paternal contributions. Filled symbols indicate L. indica, and open symbols indicate L. fauriei. Gray symbols indicate hybrid plants between L. indica and L. fauriei, whereas gradients were used to mark other Lagerstroemia species contributions. ‘Apalachee’ could not be connected to other pedigrees and is shown separately in B. ‘Low Flame’, ‘Red’, and ‘Basham's Party Pink’ are shown more than once because they were used as maternal and paternal parents of several cultivars. Cultivar names marked with asterisks were not included in this study; descriptions of all other cultivars can be found in Table 1.
Citation: Journal of the American Society for Horticultural Science J. Amer. Soc. Hort. Sci. 136, 2; 10.21273/JASHS.136.2.116
Several known hybrid cultivars cluster with L. indica. For example, ‘GAMAD I’, ‘GAMAD II’, and ‘GAMAD IV’ originated as open-pollinated seed from ‘Pocomoke’, which is a confirmed interspecific hybrid developed by the U.S. National Arboretum. Although the paternal contributions are not known, all of these cultivars should be included in the hybrid cluster. However, ‘GAMAD I’, ‘GAMAD II’, ‘GAMAD IV’, and ‘Pocomoke’ cluster within the L. indica group with ‘Chickasaw’, which is also an interspecific hybrid between L. indica and L. fauriei. ‘McFadden's Pinkie’ contains half L. subcostata genetic material and ‘Monia’ contains half L. speciosa genetic material in their background (Table 1). These two cultivars also cluster with L. indica group (Fig. 1).
Within the interspecific hybrid group, there are several subclusters that corresponded with growth habit, particularly among most dwarf and semidwarf cultivars. As mentioned, ‘GAMAD I’, ‘GAMAD II’, ‘GAMAD IV’, ‘Pocomoke’, and ‘Chickasaw’ cluster together (Fig. 1). All of these cultivars are dwarf crapemyrtles (Fig. 1). Dwarf cultivars GAMAD V and GAMAD III cluster with ‘White Chocolate’, which is semidwarf (Fig. 1). Likewise, dwarf and semidwarf cultivars Okmulgee, Rubra Compacta, and Dwarf Red cluster with each other near additional dwarf and semidwarf cultivars Velma's Royal Delight, Low Flame, and McFadden's Pinkie (Fig. 1). Clustering resulting from growth habit is likely the result of shared pedigrees, especially for cultivars named GAMAD, which are all siblings (Dirr et al., 2005).
White flower color is associated with L. fauriei. Generally, the more L. fauriei in a cultivar's genetic background, the lighter the flower color might be and this trend is evident in Table 1 in which most interspecific hybrids produce lavender or pink flowers. Presumably, red and purple flower colors are the result of specific anthocyanin accumulations that are inherited from L. indica (Zhang et al., 2008). Although not genetically verified, it is generally accepted that hybrids observed to have increased resistance to powdery mildew acquired this trait from L. fauriei (Hagan et al., 1998).
Within the last decade, almost all interspecific hybridizations were the result of crossing the best selections (i.e., named cultivars) with other cultivars. This can be seen in the U.S. National Arboretum releases in which detailed breeding notes are available (Fig. 2). Although this approach is easy and cost-effective, gains from selection can be hindered by a lack of genetic diversity in the breeding pool, especially because crapemyrtle breeding programs are restricted to only a few species. A low proportion of the genetic diversity (25.5%) is the result of differences between species and the moderate observed genetic diversity within the hybrids (average 66.5%) indicates that there is genetic diversity available for breeding. Understanding the extent and distribution of this variation within endemic and breeding populations is important for efficient germplasm preservation and accelerated breeding.
Complex crosses and high heterozygosity suggest that pedigree-based analysis may uncover markers associated with important traits in previously selected cultivars. Because SSRs are codominant and highly heterozygous, allele sizes documented here suggest that there may be an increased frequency of different alleles on homologous chromosome. Linking SSR markers with important traits may be possible because the same parents are used through multiple generations and they are often commercial cultivars that are easily accessible and have already been evaluated for important horticultural traits.
Pedigree-based analysis of the U.S. National Arboretum releases shown in Figure 2 has already identified markers potentially associated with ‘Dwarf Red’, ‘Pink Lace’, and ‘Basham's Party Pink’. Cultivars derived from ‘Dwarf Red’ seedlings include Biloxi, Zuni, Pecos, and Osage (Fig. 2). Seventeen SSR loci produced the same allele sizes found in the ‘Dwarf Red’ parent and all four derived cultivars. Two of these loci show a dramatic increase in heterozygosity when compared with the remaining 91 genotypes. For example, locus 253_254 has observed heterozygosities of 0.600 in ‘Dwarf Red’ and derived cultivars and 0.093 in the other 88 samples. Cultivars containing ‘Pink Lace’ as the parent or grandparent include Acoma, Natchez and Muskogee. ‘Osage’, ‘Miami’, ‘Wichita’, ‘Choctaw’, ‘Hopi’, ‘Sioux’, ‘Yuma’, and ‘Lipan’ were derived from ‘Pink Lace’ after additional generations (Fig. 2). Nine loci produced the same allele sizes found in ‘Pink Lace’. Likewise, 12 SSR loci produced the same allele sizes found in ‘Basham's Party Pink’ when comparing derived cultivars such as Tuskegee and Tuscarora and more distantly cultivars, Caddo, Lipan, Tonto, Arapaho, and Cheyenne (Fig. 2). However, the statistical significance of allele frequency differences was limited by the small sample size in our pedigreed samples. At this time, SSR loci producing the same allele sizes, increased heterozygosity, and/or increased allele frequency within related groups do not necessarily confirm selection pressure for those alleles.
A larger number of genotypes needs to be evaluated to compensate for the increased number of alleles (Nei et al., 1983), but once a sufficient number of genotypes and phenotypes has been evaluated, linkage results might warrant MAS. Substantial crapemyrtle populations already exist in the form of diallel populations that have been analyzed for quantitative traits (Pounders et al., 2007b). These same populations could also be sampled to look for qualitative traits associated with the SSR markers described here, especially because the parents of the diallel crosses overlap with the cultivars used in this study and the progeny were already evaluated for a range of traits (Pounders et al., 2007b). It is likely that additional associations might be uncovered as more SSR markers are screened (Wang et al., 2010).
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