Plastome-based phylogenomics elucidate relationships in rare Isoëtes species groups from the Neotropics.

The genus Isoëtes is globally distributed. Within the Neotropics, Isoëtes occurs in various habitats and ecosystems, making it an interesting case study to address phylogenetic and biogeographic questions. We sequenced and assembled plastomes and ribosomal DNA (rDNA) sequences to reconstruct phylogenetic relationships in Isoëtes from tropical regions in the Neotropics. The ploidy level of nine taxa was established to address the potential source of phylogenetic incongruence in the genus. Node ages were estimated using MCMCTree. The ancestral range estimates were conducted in BioGeoBEARS. Plastome-based phylogenies were congruent throughout distinct matrices and partition schemes, exhibiting high support for almost all nodes. Whereas, we found incongruences between the rDNA and plastome datasets. Chromosome counts identified three diploids, five tetraploids and one likely hexaploid among Neotropical species. Plastome-based node age estimates showed that the radiation of the crown Isoëtes group occurred at 20 Ma, with the diversification of the tropical American (TAA) clade taking place in the Pleistocene at 1.7 Ma. Ancestral range estimates showed that the ancestor of the TAA clade may have evolved first in the dry diagonal area in South America before reaching more humid regions. In addition, the colonization of the Brazilian semiarid region occurred three times, while the occupation of the Cerrado and Amazon regions occurred twice and once, respectively. Our study showed a large unobserved diversity within the genus in warm-dry regions in the Neotropics. Plastomes provided sufficient genomic information to establish a robust phylogenetic framework to answer evolutionary questions in Isoëtes from the Neotropics.

Unraveling the karyotype structure of the spurges Euphorbia hirta Linnaeus, 1753 and E. hyssopifolia Linnaeus, 1753 (Euphorbiaceae) using genome size estimation and heterochromatin differentiation.

Euphorbia Linnaeus, 1753 (Euphorbiaceae) is one of the most diverse and complex genera among the angiosperms, showing a huge diversity in morphologic traits and ecologic patterns. In order to improve the knowledge of the karyotype organization of Euphorbia hirta (2n = 18) and Euphorbia hyssopifolia (2n = 12), cytogenetic studies were performed by means of conventional staining with Giemsa, genome size estimations with flow cytometry, heterochromatin differentiation with chromomycin A3 (CMA) and 4',6-diamidino-2-phenylindole (DAPI) and Giemsa C-banding, fluorescent in situ hybridization (FISH) with 45S and 5S rDNA probes, and impregnation with silver nitrate (AgNO3). Our results revealed small metacentric chromosomes, CMA+/DAPI0 heterochromatin in the pericentromeric regions of all chromosomes and CMA+/DAPI- in the distal part of chromosome arms carriers of nucleolar organizing regions (NORs). The DNA content measurements revealed small genomes for both species: Euphorbia hirta with 2C = 0.77 pg and Euphorbia hyssopifolia with 2C = 1.41 pg. After FISH procedures, Euphorbia hirta, and Euphorbia hyssopifolia presented three and four pairs of terminal 45S rDNA sites, respectively, colocalizing with CMA+ heterochromatic blocks, besides only one interstitial pair of 5S rDNA signals. Additionally, the maximum number of active NORs agreed with the total number of observed 45S rDNA sites. This work represents the first analysis using FISH in the subfamily Euphorbioideae, revealing a significant number of chromosomal markers, which may be very helpful to understand evolutionary patterns among Euphorbia species.