Polyploidy in populations of well-differentiated plant species is now widely recognized (1,2). Most reports, however, are limited to a few individuals from one or several populations and thereby illustrate only a fraction of the extant genomic diversity in most species. They rarely purport populational dynamics involving Polyploidy as an evolutionary process. Nevertheless, there are recent notable exceptions and these will be utilized freely in this review of Polyploidy within species populations.

Polyploidy in species populations.

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.

The amount of DNA in the nucleus of a cell is commonly referred to as the genome size or C-value and people have been estimating this character in plants and animals for over 50 years. Today, with data available for over 7,000 species (Table 19.1), land plants (embryophytes) are the best studied of the major taxonomic groups of eukaryotes. This chapter provides an overview of what is currently known about the diversity of genome sizes encountered in land plant groups and considers how such diversity might have evolved. The chapter by Greilhuber and Leitch (2012, this volume) will discuss the impact of this diversity on plants in terms of how differences in genome size have an impact at all levels of complexity, from the nucleus to the whole organism. This chapter should also be read with reference to those by Weiss-Schneeweiss and Schneeweiss (2012, this volume) who explore intra- and inter-specific chromosome complexity across angiosperms, including polyploidy and dysploidy, Murray (2012, this volume) who discusses gymnosperm chromosomes, and Barker (2012, this volume) who considers the chromosomes of monilophytes and lycophytes.

Genome Size Diversity and Evolution in Land Plants

Meiosis in homosporous ferns allows chromosomes to pair within homoeologous sets rather than restricting pairing to homologs; this allows homozygous spore mother cells to give rise to genetically heterogeneous meiospores. It is envisioned that such a meiotic system has evolved in homosporous ferns in response to a life cycle which allows the attainment of total homozygosity in one generation of selfing.

Sexual and subsexual systems in homosporous pteridophytes: a new hypothesis

The evolution of sexual reproduction has seen a recent and major resurgence as a topic of interest. Many authors (e.g. Ghiselin 1974, Williams 1975, Maynard Smith 1978, Bell 1982, Shields 1982, Bull 1983, Michod and Levin 1988) have refined the now-familiar arguments that generally cast sexual reproduction as the alternative to the asexual production of genetic clones. As theorists have moved further from the usual cytogenetic models (i.e. humans, mice, fruitflies and maize), we have learned that the many strange genetic systems and breeding biologies of plants, animals and protists blur the distinction between “sexual” and “asexual”.

Cytogenetics of Holokinetic Chromosomes and Inverted Meiosis: Keys to the Evolutionary Success of Mites, with Generalizations on Eukaryotes

Cytomorphological variation in Selaginella

Stable telocentrics are present in a number of plant species but rarely form a major part of a complement and almost never constitute its whole. T. micrantha (2n=24) is an exceptional, possibly unique, plant in which all the chromosomes are of this type. They fall into three size categories (L, M, and S) each forming quadrivalents with frequencies positively correlated with chromosome length: it is concluded that the species is autotetraploid. Pairing and segregational errors result in high frequencies of gametic aneuploidy — a fact which is taken to reflect the vegetative vigour of the species and a low level of selection for sexual fertility. The cytological features determined in this study provide a basis for discussing the significance and mode of evolution of the complement.

Carrol Colden

Papers

Cytomorphological variation in Selaginella

The telocentric complement of Tradescantia micrantha

Contributions to the cytotaxonomy of the Commelinaceae: I. Tradescantia geniculata and its allies