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Chromosome-Level Genome Assembly of Solanum carolinense.

Horsenettle (Solanum carolinense L.) is a noxious weed widely distributed across North America and increasingly invasive in other regions. Its strong environmental adaptability, complex defense strategies, and distinctive reproductive traits make it an important model for studying plant-herbivore coevolution. However, the absence of high-quality genomic resources has limited deeper investigation into its adaptive evolutionary mechanisms. In this study, we generated a chromosome-level reference genome assembly for S. carolinense using an integrated approach combining PacBio HiFi long-read sequencing, Illumina second-generation sequencing, and Hi-C chromatin interaction scaffolding. The final genome assembly had a total length of 915.40 Mb, with a contig N50 of 51.06 Mb and a scaffold N50 of 73.17 Mb; 96.05% of the sequences were successfully anchored onto 12 pseudochromosomes. The genome was characterized by a high proportion of repetitive sequences (73.64%) and substantial heterozygosity (1.13%), consistent with a highly repetitive and moderately high heterozygous genome. BUSCO analysis indicated that the chromosome-level genome assembly of S. carolinense reached a completeness score of 94.8%. A total of 32,206 protein-coding genes were annotated, of which 97.95% received functional annotations. The evaluation of the annotated protein-coding gene set returned a completeness value of 94.9%. This reference genome provides a valuable resource for advancing research on the adaptive evolution of weedy Solanaceae species, supports the development of more effective management strategies for this troublesome species, and offers a technical reference for assembling other highly heterozygous weed genomes.

Solanum carolinense↗

S-allele sequence diversity in natural populations of Solanum carolinense (Horsenettle).

S-allele diversity in Solanum carolinense was surveyed in two natural populations, located in Tennessee and North Carolina, with a molecular assay to determine the genotype of individual plants. A total of 13 different S-alleles were identified and sequenced. There is high overlap between the two populations sampled, with 10 alleles shared in common, one allele found only in Tennessee, and two found only in North Carolina. The number of alleles in this species appears to be extremely low compared with other species with gametophytic self-incompatibility. Sequence comparisons show that most alleles are extremely different one from another in their primary sequence and a phylogenetic analysis indicates extensive trans-specific evolution of S-lineages. In addition, some alleles appear to be derived much more recently. The implications of these observations are discussed in the light of recent theoretical results on S-allele population diversity and persistence.

Alleles↗

Carolinoside: a phytosteroidal glycoside from Solanum carolinense.

The glycoside of a new class of phytosteroids has been isolated from Solanum carolinense. The steroidal aglycone (carolinone) is alkylated at C-3 and is identified as C-[(2,4,5-trideoxy-3-keto-4,5-dehydro)-pentulopyranosyl]-(5----3)- (13,14- seco-14 beta,17 alpha-dihydroxy) estrogen. The hydrolytic labile glycosyl moiety is identified as O-(beta-D-glucopyranosyl) (1----1)-[L-(2,6-dideoxy-3-C-methyl)- arabinopyranose]. The linkage of this disaccharide in the steroidal glycoside (carolinoside) is shown to be O-(alpha-pentulopyranosyl)- (1----4)-O-(beta-L-arabinopyranosyl)-(1----1)-D-glucopyranose. Carolinoside occurs at concentrations of 10(-7)-10(-6) M in leaf tissue and was shown to be the host plant specific feeding induction factor for Manduca sexta.

Glycosides↗

Effect of nitrogen and water treatment on leaf chemistry in horsenettle (Solanum carolinense), and relationship to herbivory by flea beetles (Epitrix spp.) and tobacco hornworm (Manduca sexta).

We studied the interaction between plants (horsenettle; Solanum carolinense) and herbivorous insects (flea beetles; Epitrix spp., and tobacco hornworm; Manduca sexta) by focusing on three questions: (1) Does variation in nitrogen availability affect leaf chemistry as predicted by the carbon-nutrient balance (CNB) hypothesis? (2) Does variation in plant treatment and leaf chemistry affect insect feeding? (3) Is there an interaction between the insect herbivores that is mediated by variation in leaf chemistry? For three successive years (1998-2001), we grew a set of clones of 10 maternal plants under two nitrogen treatments and two water treatments. For each plant in the summer of 2000, we assayed herbivory by hornworms in both indoor (detached leaf) and outdoor (attached leaf) assays, as well as ambient flea beetle damage. Estimates of leaf material consumed were made via analysis of digitized leaf images. We also assayed leaves for total protein, phenolic, and glycoalkaloid content, and for trypsin inhibitor, polyphenol oxidase, and peroxidase activity. Despite strong effects of nitrogen treatment on growth and reproduction, only total protein responded as predicted by CNB. Leaf phenolic levels were increased by nitrogen treatment, polyphenol oxidase activity was decreased, and other leaf parameters were unaffected. Neither hornworm nor flea beetle herbivory could be related to plant treatment or genotype or to variation in any of the six leaf chemical parameters. A negative relationship between flea beetle and hornworm herbivory was found, but was not apparently mediated by any of the measured leaf chemicals. Because leaf resistance was maintained in low nitrogen plants at the apparent expense of growth and reproduction, our results support the concept of a fitness cost of defense, as predicted by the optimal defense hypothesis.

Adaptation, Physiological↗

Fertility, root reserves and the cost of inducible defenses in the perennial plant Solanum carolinense.

We examined the relationship between internal resources (root reserves), external resources (soil fertility), and allocation to defense vs. growth in the clonal, perennial herb Solanum carolinense. In a short-term (9 d) greenhouse experiment, plants were treated once with jasmonic acid (JA) to determine if polyphenols and glycoalkaloids were inducible by simulated herbivory. In a longer-term (4 wk) greenhouse experiment, we measured the cost, in terms of growth, of treatment with JA every 3 d, to determine if the induced response was due more to carbon limitation or nitrogen limitation. We manipulated the resources available to the plants by varying soil fertility and the size of root cuttings from which plants were grown, and assessed how different resource levels affected the growth and production of polyphenols and alkaloids under JA treatment or control conditions. In the short term, JA increased the concentration of polyphenols in both above- and below-ground plant parts, as well as alkaloid concentrations in the roots. In the long term, the only significant secondary chemistry response to JA was an increased polyphenol concentration in above ground tissues. The total amount of polyphenols produced was the same for JA and control plants, indicating that the higher concentration was a result of the lower biomass of treated plants. In contrast, alkaloid concentrations in plants treated with JA for 4 wk did not differ from controls, but JA-treated plants contained lower total amounts of alkaloids in above ground tissues, as a result of decreased growth. Fertilizer level and root cutting size had effects on growth and the production of secondary compounds and influenced the cost of induction. Plants grown under high fertility had a greater reduction in growth in response to JA than plants grown under low fertility, indicating a greater trade-off between growth and defense for high fertility plants. Plants from larger root cuttings grew bigger without any reduction in the concentration of polyphenols and alkaloids. We demonstrated that the phenotype of S. carolinense was plastic in response to simulated herbivory, fertility level, and root cutting size, and that there was a significant growth cost to induction that varied with the environment and appears to be due in large part to the allocation of limited carbon reserves.

Agriculture↗

Sheltered load associated with S-alleles in Solanum carolinense.

Bud pollinations allowed me to examine the effects of homozygosity at loci in the area of suppressed recombination around the S-locus in Solanum carolinense, whose S-alleles show surprisingly low diversification rates. The total number of seeds produced was lower for incompatible than compatible pollinations, revealing that self-incompatibility was only somewhat overcome by bud pollination. However, low seed set in incompatible crosses was not due solely to the incompatibility response; crosses between distinct plants sharing the same alleles at the S-locus led to dramatically high seed abortion, nearly equal to that found upon selfing. An excess of heterozygotes in the surviving progeny supports the supposition that these high abortion rates are due to sheltered load, that is, previously unexpressed load accumulated due to enforced heterozygosity and recombination suppression around the S-locus. Of the seven alleles examined in total, two showed evidence of severe load and five did not. The magnitude of load was consistent with terminal branch length in some, but not all, cases.

Alleles↗

Defense tradeoffs in fleshy fruits: effects of resource variation on growth, reproduction, and fruit secondary chemistry in Solanum carolinense.

A set of clones of 10 maternal plants was grown for three successive years (1998-2000) under two nitrogen treatments and two water treatments. Path analysis revealed strong direct and indirect effects of nitrogen treatment on growth and reproduction, but fruit morphological and chemical variables were not strongly affected. Fruit pulp chemistry varied only slightly across treatments despite the large differences in growth and reproduction associated with resource variation. Leaf and ripe fruit chemical contents were not significantly correlated across treatments, and maternal plants, and leaf chemical variables did not help explain fruit chemical variation when included as covariates in ANCOVA analyses, suggesting no physiological constraints of leaf chemistry on ripe fruit chemistry. Results suggest that, while maternal plants may vary somewhat in fruit chemistry, and fruit chemistry may vary somewhat depending upon environmental conditions, levels of primary and secondary metabolites within fruits are not best explained by supply-side hypotheses. Ripe fruit chemistry remained relatively constant in the face of drastically changing resource levels, suggesting an adaptive function and supporting the Defense Tradeoff hypothesis. Fruit quality, both in terms of nutritional make-up and putative defensive properties, was maintained despite strong effects on plant growth and reproduction. Because glycoalkaloids are general defense compounds, we conclude that ripe fruit chemistry most likely reflects a balance between selection for attraction of seed dispersers and defense against pests and pathogens.

Adaptation, Physiological↗

Pollen performance before and during the autotrophic-heterotrophic transition of pollen tube growth.

For species with bicellular pollen, the attrition of pollen tubes is often greatest where the style narrows at the transition between stigmatic tissue and the transmitting tissue of the style. In this region, the tubes switch from predominantly autotrophic to predominantly heterotrophic growth, the generative cell divides, the first callose plugs are produced, and, in species with RNase-type self-incompatibility (SI), incompatible tubes are arrested. We review the literature and present new findings concerning the genetic, environmental and stylar influences on the performance of pollen before and during the autotrophic-heterotrophic transition of pollen tube growth. We found that the ability of the paternal sporophyte to provision its pollen during development significantly influences pollen performance during the autotrophic growth phase. Consequently, under conditions of pollen competition, pollen selection during the autotrophic phase is acting on the phenotype of the paternal sporophyte. In a field experiment, using Cucurbita pepo, we found broad-sense heritable variation for herbivore-pathogen resistance, and that the most resistant families produced larger and better performing pollen when the paternal sporophytes were not protected by insecticides, indicating that selection during the autotrophic phase can act on traits that are not expressed by the microgametophyte. In a study of a weedy SI species, Solanum carolinense, we found that the ability of the styles to arrest self-pollen tubes at the autotrophic-heterotrophic transition changes with floral age and the presence of developing fruits. These findings have important implications for selection at the level of the microgametophyte and the evolution of mating systems of plants.

Cucurbita↗

Historical events and allelic polymorphism at the gametophytic self-incompatibility locus in Solanaceae.

The historical migration rate of a species is often difficult to estimate with neutral markers, because the relationship between the turnover time of the markers and the age of the species commonly remains unknown. Compared with neutral markers, the plant self-incompatibility locus (S) provides a much better source of data for migration-rate estimation due to its high allelic polymorphism and antiquity. Here, the results from extensive surveys of S alleles in two wild solanaceous species, Solanum carolinense and Physalis longifolia, indicate that historical migration rates have differed significantly between the species; the higher migration rate found in S. carolinense appears to have interacted with the balancing selection at the S locus to result in fewer S alleles being maintained in the species. Historical population growth rates estimated via a modified coalescent approach also suggest a faster growing population for S. carolinense than for P. longifolia, which would have further widened their interspecific difference in S-allelle polymorphism. These historical factors may have reduced the probability of new S alleles to prevailing in S. carolinense, leaving old ones segregating at the S locus with little signature of positive selection being currently detectable.

Alleles↗