Research Highlights: High variation of genetic diversity and differentiation among 27 seed sources within 14 natural provenances and 13 land race samples of Casuarina equisetifolia were found. High proportions of monoecious individuals may be present in some populations, as indicated by severe heterozytote deficiency and inbreeding found in many provenances and land races. The most probable origins of the land races were inferred according to the values of pairwise provenance differentiation and Nei’s genetic distances. Targeted introductions and testing of unrelated new accessions of C. equisetifolia from the Pacific and Philippines was proposed to identify Ralstonia-resistant genotypes. Background and Objectives: Casuarina equisetifolia was introduced to China a hundred years ago and has become a critically important tree species in coastal protection since the 1950s. Despite its importance, patterns of genetic variation, genetic relationships among natural provenances and probable origins of the land races remain unresolved. This has become a concern in China where Ralstonia solanacearum bacterial wilt has devastated plantations that are known to be from a narrow genetic base that urgently needs to be broadened. Materials and Methods: Fourteen natural provenances from Australia, Pacific islands and Southeast Asia, and 13 land race samples from parts of Asia and Africa outside the natural range were genotyped using 13 SSR (Simple Sequence Repeats) markers to characterize their allelic variation and genetic relationship. Results: Significant genetic diversity and differentiation among 27 seed sources within 14 provenances and 13 land race samples of C. equisetifolia was indicated. Significant heterozygote deficiency and inbreeding was indicated for a number of provenances, perhaps indicating a high proportion of monoecious parents in these populations. The most probable origins of the land races of the introduced countries were suggested according to the values of pairwise provenance differentiation (F_ST) and Nei’s genetic distances. Conclusions: We found significant genetic diversity and genetic differentiation among seed sources of C. equisetifolia. While individual land races do not appear to lack diversity, we were able to infer the origins of some, allowing targeted introductions of unrelated material to be made. In the case of the Chinese land race, targeting and testing new accessions from the Pacific and the Philippines may be a good strategy to identify Ralstonia-resistant genotypes.

The Tabebuia alliance is widely planted in tropical and subtropical countries. However, the lack of accurate taxonomic information for many commercial cultivars leads to commercial disputes. We sought to clarify the background of commercial cultivars in China and to provide a robust basis for patent protection. We collected 82 Tabebuia cultivars and 12 reference individuals, then developed large-scale single nucleotide polymorphism (SNP) markers using genotyping-by-sequencing. We obtained 73,219 SNPs from the test samples, which represented a substantial increase compared with previous studies of Tabebuia species. Based on these SNPs and a clustering analysis, we detected six species among the samples, as well as an uncertain population which may be a hybrid between Handroanthus impetiginosus (Mart. ex DC) Mattos and H. heptaphyllus (Vell.) Mattos. An analysis of genetic diversity to assess germplasm resources indicated that excessive heterozygosity of these cultivars in China, which may explain the genetic basis of “novel-preferred” selection in ornamental plants or a narrow genetic background. After rigorous selection, we identified numerous high-quality molecular markers for cultivar identification, then constructed a concise fingerprint with 54 SNPs. Our work clarifies the background information of commercial cultivars of Tabebuia in China and provides information for cultivar identification. It will serve as an important technical resource for genetic studies, and it will help to protect patents that involve Tabebuia species.

Casuarina cunninghamiana Miq. naturally occurs in eastern Australia from New South Wales to north Queensland. After being introduced to China, it has become an important tree species of ecological shelter plantations in coastal areas of southern China. In this study, the complete chloroplast (cp) genome of C. cunninghamiana was sequenced and analyzed based on the Illumina NovaSeq 6000 platform. The cp genome of C. cunninghamiana was found to be 15,6129 bp in length, including a large single copy (LSC) region of 86,200 bp and a small single copy (SSC) region of 18,457 bp, which were separated by two inverted repeats (IRs) of 25,736 bp. The cp genome contains 132 genes, consisting of 87 protein-coding genes, 37 tRNA genes, and eight rRNA genes. The overall GC content of the cp genome was 36.34%. The phylogenetic analyses indicated that C. cunninghamiana was closely related to C. glauca and C. equisetifolia and clustered with 4 Betulaceae species.