How small and constrained is the genome size of angiosperm woody species

Abstract

Angiosperm hardwood species are generally considered to show an average smaller genome size with a narrow range of variation than their herbaceous counterparts. Various explanations pertaining to limitations of cell size exerted by wood fibers, the requirement of smaller stomata, longer generation time, large population size, etc., have been put forward to account for their small and constrained genome size. Yet studies done in the past several years show that genomically as well as evolutionarily, hardwoods are as diverse and active as their herbaceous counterparts. This is entirely supported by the presence of well developed inter and intraspecific polyploid series and natural triploidy in many genera. Polyploidy, in some instances has been shown to confer adaptability to arid and salt stress conditions and in colonization of new areas. Moreover, hardwoods also show reasonable amenability to the induced polyploidy which abruptly changes the balance between nuclear and cell size. Polyploidy has been induced in many hardwoods to restore fertility in interspecific hybrids and for the production of triploids.Furthermore, some cases studied show that genome size variation in hardwoods can be as variable as that of herbaceous species. Genome size has been shown to vary remarkably both at homoploid level as well as by polyploidy in certain genera. In the same way, the genome size is not correlated with the habit in certain groups having both herbaceous and woody taxa. This point is further proved by the presence of secondary and insular woody habit in certain cases where either the transition to woodiness is not followed by any diminution in the genome size, or the genome size of insular woody species may be even more than that of the congeneric herbaceous species. This shows that woody habit does not by itself put any constraints on the genome size either at homoploid or at polyploidy levels. The genome size in fact, not only varies significantly in many congeneric woody species but also may not show any correlation with the habit when woody and herbaceous species are compared in some narrow taxonomic groups studied.

Genome sequencing of Syzygium cumini (jamun) reveals adaptive evolution in secondary metabolism pathways associated with its medicinal properties

“…We also report the transcriptome assembly of S. cumini for the first time in this study. We inferred S. cumini genome to be tetraploid in this study, whereas previous studies have also shown the existence of intraspecific polyploidy (ranging from 2x to 6x) in S. cumini species (Ohri, 2015;Sharma et al, 2020). The phylogenetic position of S. cumini was resolved with respect to 17 other Eudicot orders, and comparative evolutionary analyses showed the key plant secondary metabolism pathways, such as the phenylpropanoid-flavonoid (PF) biosynthesis pathway, were adaptively evolved in this Syzygium species, which are responsible for the immense medicinal properties of this tree.…”

“…It is only the third and till date the largest genome to be sequenced from the largest tree genus containing approximately 1,200 species. S. cumini was previously reported to show intraspecific polyploidy compared to S. aromaticum and S. grande ( Ohri, 2015 ). Our analyses using two independent approaches to estimate the genomic ploidy also confirmed the tetraploidy in S. cumini genome.…”

Polyploidy in Gymnosperms-A Reappraisal

“…Remarkably, besides the classic examples of Sequoia sempervirens and Fitzroya cupressoides, a very high incidence of polyploidy has been reported in Ephedra (76.0 %) and Juniperus (22.3 %) as also the recent discovery of spontaneous production and sustenance of various polyploid forms under cultivation in Ginkgo (Smarda et al 2018). In sharp contrast to this angiosperm hardwoods not only show high basic numbers resulting from paleopolyploidy but also have well-developed polyploid series and complex dysploid number variation across various families and genera (Ohri 2015). Dysploidy is also observed in some gymnosperm taxa e.g.…”

Variation and Evolution of Genome Size in Gymnosperms

“…In contrast to the hardwoods which show extensive inter-and intraspecific polyploid series and palaeoploidy (Ohri 2015), gymnosperms possess a very narrow range of somatic chromosome numbers (2n=14-66), conserved karyotypes within various families and genera Ohri 2020 a,b, Ohri 2021b) and rare instances of recent polyploidy (Khoshoo 1959, Ahuja 2005, Ohri, 2021a. Only some genera like Zamia, Pseudotsuga, Pseudolarix, Prumnopitys, Halocarpus, Podocarpus exhibit dysploid numbers Ohri 2020a, Ohri 2021b).…”