Summary
The gross form of the nuclear genome varies greatly among plant species in both anatomy and genetic organization. Chromosome number (ft) ranges from 2 to over 600, and ploidy from 1 to over 20. The amount of DNA in the unreplicated haplophase genome (the 1C value) differs by more than 2500-fold among angiosperms. Although it has been questioned since the 1930s whether such variation is of adaptive significance and whether it is related, perhaps causally, with environmental factors, no direct or causal links have yet been found. However, variation in DNA C-value has far-reaching biological consequences and can be of considerable adaptive and hence ecological significance. Strikingly precise interspecific relationships exist between DNA C-value and many diverse phenotypic characters at the cellular level, and DNA can affect the phenotype in two ways, firstly by expression of its genie content and, secondly, by the biophysical effects of its mass and volume, the latter defined as nucleotypic effects. Nucleotypic variation in DNA C-value sets absolute limits to both the minimum size and mass of the basic unit of plant anatomy (i.e. the cell) and the minumum time needed to produce a similar cell with newly synthesized organic molecules. Moreover, in complex multicellular vascular plants, such effects at successive cell cycles are additive, so that DNA C-value influences many characters, including growth rate, seed weight, minimum generation time and type of life-cycle. Thus, the nucleotype profoundly affects where, when and how plants grow. Selection for a particular genomic form acting on its spatial or temporal consequences may occur at various levels ranging from the cell to the whole organism and may operate throughout the life-cycle or at just one stage. DNA C-value is often indirectly related to environmental factors which determine time-limited environments via selection acting on the temporal phenotypic consequences of nucleotypic variation. However, in the case of radio-sensitivity, selection for a low DNA C-value may act directly on the nucleotype itself, as the size of the nuclear DNA target directly affects the ability of the plant to survive.

Variation in genomic form in plants and its ecological implications

Impatiens glandulifera Royle (Impatiens roylei Walp.).

Publisher Summary This chapter discusses the B-chromosome systems in flowering plants and animal species. The term B chromosome is introduced to describe extra chromosome which have little if any effect on visible characters of the plant, which are not homologous with the A chromosomes of the normal complement, and which are extremely irregular in their meiotic distribution. B Chromosomes are now known under a variety of different names, including supernumerary and accessory, which are the most common alternatives. B chromosomes are dispensable and nonhomologous with A chromosomes. These two characteristics above all others most sharply differentiate them from members of the basic A-chromosome complement. The distribution of B chromosome in plant and animal species is discussed. In many plants and animals the B chromosomes are completely stable during the cell cycle and are inherited in a constant and unchanging form along with the A chromosomes. Some plant species have an apparent order in their instability, which results in exclusion and/or accumulation of B chromosomes in specific tissues and organs. The significance of B-chromosome effects in flowering plants and animals is discussed.

B-Chromosome Systems in Flowering Plants and Animal Species

Sequences of the internal transcribed spacers (ITS) of nuclear ribosomal DNA were acquired for 112 species of Balsaminaceae worldwide and five species of its closest relatives Marcgraviaceae and Tetrameristaceae. Phylogenetic analyses applying parsimony and distance estimates confirmed the monophyly of Balsaminaceae and suggest the monophyly of Impatiens. Within Impatiens, a few clades are recognized with strong support. Two of the most important clades are the spurless Madagascan endemic group, and the one comprising species with broadly fusiform fruits and the basic chromosome number x = 8, that shows a Southeast Asia, southern India, Africa, and Madagascar connection. Despite recognition of several strongly supported small lineages, ITS data alone could.not resolve relationships among most of the lineages with confident support values. ITS phylogenies are therefore of limited taxonomic value for Impatiens. However, ITS phylogenies do reveal that extant Impatiens species are of Southeast Asian origin, from where dispersals to boreal Eurasia and North America, to central Asia and eastern Europe via the Himalayas, and to India and Africa have occurred. The Madagascan Impatiens show an African origin. Molecular phylogenies suggest the ancestral basic chromosome number to be x = 10, and the spurred flowers and elongated linear fruits to be plesiomorphic states in Impatiens. A predominantly descending dysploid chromosome evolution, following dispersal of the clade with broadly fusiform fruits from Southeast Asia to India, Africa, and Madagascar, is also suggested.

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Phylogeny and biogeography of Balsaminaceae inferred from ITS sequences

A revised generic synopsis of sub-Saharan Hyacinthaceae is presented, based on a molecular phylogenetic analysis of the family. Generic rank is accorded only to reciprocally monophyletic clades that can be distinguished by recognizable morphological discontinuities, thereby permitting an appropriate generic assignment of species not included in the analysis. Three subfamilies are recognized within the region. Subfamily Ornithogaloideae, characterized by flattened or angular seeds with tightly adhering testa, is considered to include the single genus Ornithogalum, which is expanded to include the genera Albuca, Dipcadi, Galtonia, Neopatersonia and Pseudogaltonia. Recognizing any of these segregates at generic level renders the genus Ornithogalum polyphyletic, while subdivision of Ornithogalum into smaller, morphologically distinguishable segregates in order to preserve the monophyly of each is not possible. Subfamily Urgineoideae, characterized by flattened or winged seeds with brittle, loosely adhering testa, comprises the two mainland African genera Bowiea and Drimia. The latter is well circumscribed by its deciduous, short-lived perianth and includes the previously recognized genera Litanthus, Rhadamanthus, Schizobasis and Tenicroa. The monotypic Madagascan Igidia is provisionally included in the subfamily as a third genus on the basis of its seeds, pending molecular confirmation of its relationships. Subfamily Hyacinthoideae resolves into three clades, distinguished as tribes Hyacintheae (strictly northern hemisphere and not treated further), Massonieae and Pseudoprospereae tribus nov. Full descriptions and a key to their identification are provided for all genera. New combinations reflecting the generic circumscriptions adopted here are made for most African and all Indian and Madagascan species.

A revised generic synopsis of hyacinthaceae in sub-saharan africa, based on molecular evidence, including new combinations and the new tribe pseudoprospereae

Many plant genera are known to include species with different basic chromosome numbers; such occurrences are generally of interest, but more particularly when cytological variation of this sort is correlated with changes in growth form, reproductive behaviour or distribution. Impatiens L. is a genus of this type-one in which both variation in basic number and the incidence of polyploidy show decided geographical patterns. Recent studies at the Jodrell Laboratory, Kew, make a significant contribution to this knowledge and form the basis for this first report. Impatiens is essentially a montane genus of the Old World containing over 500 species, half of which occur in the sub-continent of India. It is common in Burma and Africa but less so in SE. Asia and is relatively rare in northern Asia, Europe and North America. There is no taxonomic monograph of the genus and the only significant attempts at classification of the species are those of Hooker (1874-5) and again Hooker (1904-6). The classifications are markedly different in the two works due to the discovery of many more species in the intervening years. It had also become more apparent that endemism was prevalent and this resulted in a primarily phytogeographic treatment of the species in the later work. More recently it has been estimated that as many as 91 per cent. of the Indian species are endemics (Chatterjee, 1940: 44)Cytological studies have also been few in number and, prior to the present study, only twenty-three species had been chromosome counted. The latest and most substantial contributions have been made by Khoshoo (1955, 1956, 1957) who, in his I957 paper, lists and reviews all the previous works. At that time two main cytological characteristics of the genus emerged, namely that it was exclusively diploid and that the basic numbers of the species showed considerable variation (x = 6, 7, 8, Io). Although limited, the combined data also suggested that x = 8 might be typical for African species.

The Cytogeography of Impatiens L. (Balsaminaceae)

Knowledge of chromosome shape and size in related taxa can be of fundamental importance to the evolutionist and taxonomist alike. The study of karyotypes is particularly rewarding in such a family as the Liliaceae, where large chromosomes and frequent bimodality in size within complements, make possible the determination of the progress of chromosome change and its consequences. For many genera however we still await the basic information on karyotype characteristics of constituent species before attempting to assess the evolutionary processes and trends. For the Liliaceae the description of the chromosome complements of the following twenty-five species will add more grist to the mill.

The Chromosomes of the Liliaceae: I: The Karyotypes of Twenty-Five Tropical Species

SUMMARYThe somatic chromosomes of 28 accessions of Crinum have been studies. Ten of these were polyploid (3x, 4x, 6x, 8x) and the remainder diploid. B chromosomes were present in seven plants. Aneusomatic variation does not occur to any significant extent.The common basic number is x = 11 but accessions, based on x = 12 derive from the Accra Plains of Ghana. Comparative chromosome morphology shows that a uniform pattern prevails throughout the x = 11 species but that of the x = 12 plants is quite distinct and includes three pairs of telocentrics. There is every indication that this unique karyotype is closely correlated with the distribution of the species in an area of particular ecological interest.

https://www.tandfonline.com/doi/pdf/10.1080/00087114.1967.10796255

Chromosome Evolution in the Genus Crinum

In a recent discussion of the difficulties encountered in attempting to arrive at a wholly satisfactory circumscription of the genus Euphorbia L., Webster has pointed out that a shortage of cytological data, as well as basic morphological information, has been a serious hinderance to progress (Webster, I967). Such chromosome studies as have been made are mostly in the subgenus Esula and African succulents, and no data is available for 18 out of the 27 sections recognized by Boissier. This deficiency is particularly unfortunate in a group where cytology has proved its taxonomic usefulness. Its correction is thus highly desirable and, as a contribution to this end, we describe in this paper the cytology of a substantial number of species in the African genera Monadenium Pax and Synadenium Pax. Both genera are allied to Euphorbia but may be distinguished from it by their characteristic zygomorphic cyathium, and from each other in the structure of the involucral gland. In Synadenium the latter is a single rim-like organ but in Monadenium it is open at the front (horseshoe-shaped). Monadenium has been monographed by Bally (1961) who describes 47 species and a number of forms and varieties, almost all of which are confined to tropical East Africa only one extending beyond the tropic of Capricorn and three into West Africa. They exhibit a wide range of growth-forms varying from dwarf stemless plants to succulents with stout tubercular stems and branches which occasionally reach the dimension of trees. Synadenium has not been monographed and is consequently less well known. However 18 species are listed and all are large shrubs or trees. Most taxonomists appear to accept Monadenium and Synadenium as good genera but Webster has demurred, seeing them as 'evidently allied to Euphorbia sect. Tirucalli and are scarcely more distinctive than are many of Boissier's sections which are usually retained within Euphorbia' (Webster 1967). His opinion like those of all previous taxonomists is based on the morphological aspects of the plants without the benefit of any significant cytological data. No chromosome studies have been reported for Monadenium whilst for Synadenium only two counts (2n = 36) for S. grantii have been published (Raman & Kesavaran, 1963, Datta, 1967). There is no consistent r6le which cytology can play in defining generic limits, nevertheless, in demonstrating fundamental differences or similarities, they can call attention to unsuspected situations or vindicate existing opinion. Thus the data which we present below must be taken into consideration in all future discussions of the status of the species.

https://www.jstor.org/stable/info/4117196

J. B. Smith

Papers

The Cytogeography of Impatiens L. (Balsaminaceae)

The Chromosomes of the Liliaceae: I: The Karyotypes of Twenty-Five Tropical Species

Chromosome Evolution in the Genus Crinum

The Chromosome Identity of Monadenium Pax and Synadenium Pax (Euphorbiaceae)