https://www.plantcytogenomics.org/sites/default/files/Schubert%2C%20Lysak%2Ckaryotype%20evol_constraints%2CTiG2011.pdf

Interpretation of karyotype evolution should consider chromosome structural constraints.

summary
This review of supernumerary B chromosome systems in flowering plants deals mainly with work published in the last 15 yr, hut also includes older material which has not hitherto been presented systematically. The term B chromosome (B) is defined, and there is an introductory overview dealing with general characteristics and the significance of Bs as a widespread chromosome polymorphism. Detailed sections are then presented covering the occurrence of Bs in different taxa, their structure and molecular organization, their irregular modes of inheritance, their phenotypic effects, population dynamics and origin. Particular attention is paid to the research growth points in molecular analysis of the structure and genome organization of Bs, and to transmission genotypes in the context of their adaptive versus their selfish properties. Where appropriate, reference is also made to likely future lines of research, and also to the usefulness of B chromosomes in genetic analysis and as model systems to study general phenomena of genome organization and evolution, nuclear physiology and architecture, chromosome polymorphism and selfish DNA.

B chromosomes in plants

Evolution of DNA amounts across land plants (embryophyta)

Polyploidy, the multiplication of chromosome sets above the diploid state, is arguably as prevalent in plants as any other eukaryotic group. Over the past century, polyploidy has been linked with particular taxonomic groups (flowering plants, ferns), northern latitudes and extreme environments, and life histories such as perenniality, asexuality and self-fertilization. Here we review the current state of evidence for these patterns and their interpretations. Our understanding of the incidence of polyploidy in plants is still in flux due to a progression of advances in cytogenetics, flow cytometry, phylogenetics and genomics. We find polyploidy to be highly heterogeneous among plant groups and estimates of intraspecific variation and hybrid forms are increasing with high throughput analyses. Surprisingly, many accepted trends between polyploidy and geography, mating system, and comparative rates of species diversification have become less strong or conflicting with the development of new procedures, in particular with phylogenetic approaches. Our review suggests that while some patterns remain strong, many are not universal, suggesting there may be no unified explanation for the success of polyploids in plants.

The Incidence of Polyploidy in Natural Plant Populations: Major Patterns and Evolutionary Processes

Contents
 	Summary	629	
I.	Introduction – overview	629	
II.	Genome structures	631	
III.	Factors that have shaped the genomes of angiosperms and gymnosperms	640	
IV.	The future of genomic studies in seed plants	643	
 	Acknowledgements	643	
 	References	643	


Summary
The large-scale replacement of gymnosperms by angiosperms in many ecological niches over time and the huge disparity in species numbers have led scientists to explore factors (e.g. polyploidy, developmental systems, floral evolution) that may have contributed to the astonishing rise of angiosperm diversity. Here, we explore genomic and ecological factors influencing seed plant genomes. This is timely given the recent surge in genomic data. We compare and contrast the genomic structure and evolution of angiosperms and gymnosperms and find that angiosperm genomes are more dynamic and diverse, particularly amongst the herbaceous species. Gymnosperms typically have reduced frequencies of a number of processes (e.g. polyploidy) that have shaped the genomes of other vascular plants and have alternative mechanisms to suppress genome dynamism (e.g. epigenetics and activity of transposable elements). Furthermore, the presence of several characters in angiosperms (e.g. herbaceous habit, short minimum generation time) has enabled them to exploit new niches and to be viable with small population sizes, where the power of genetic drift can outweigh that of selection. Together these processes have led to increased rates of genetic divergence and faster fixation times of variation in many angiosperms compared with gymnosperms.

Ecological and genetic factors linked to contrasting genome dynamics in seed plants.

Chromosome evolution in the genus Ophioglossum L.

Followings are other important observations:1. Occurrence of supernumerary chromosomes of very small size (1.4μ); i.e. microchromosomes are recorded in O. nudicaule, O. gramineum, O. sp. and O. thermale.2. A peculiar phenomenon of linear attachment of bivalents i.e., interchromosomal connections is observed for the first time in Ophioglossum.3. Plants of O. thermale growing intermixed with O. sp. are characterised by ca. 360 bivalents, which is a new count for the species.

Chromosomal elimination in natural populations of Ophioglossum.

Chemosystematics of Indian Ophioglossum L.

Following are the impotant observations:1. Physico-chemical properties of fatty oil of Ophioglossum costatum has been determined for the first times.2. The present study is a pioneer attempt to see the effect of oil of O. costatum on mitotic cell division of A. sativum and V. faba.3. The mitotic index was seen to decrease gradually as the time of treatment was increased.4. Prolonged treatment causing the death and deorganisation of actively dividing cells.5. This oil has the potential use in anti-cancer chemotherapeutic agent.

Effect of Ophioglossum Oil on Mitosis in Allium sativum and Vicia faba

Following are the important observations:1. The present study is a pioneer attempt to see the effect of magnetic field on the meiotic cell division of a lower vascular plant.2. The strength of magnetic field applied is 1.6×105 Gauss for one hour, which is of greater strength and for longer duration.3. The abnormalities can be explained on the bases of the effect of magnetic field in the change of tetrahedronal linking of C-C bond, viscosity, surface tension and circadian rhythm.

Sharda Khandelwal

Papers

Chromosome evolution in the genus Ophioglossum L.

Chromosomal elimination in natural populations of Ophioglossum.

Chemosystematics of Indian Ophioglossum L.

Effect of Ophioglossum Oil on Mitosis in Allium sativum and Vicia faba

Effect of Magnetic Field on Chromosomal Behaviour of Ophioglossum Linn