Diverse Arrangements Research Articles

Experimental Hybridization and Chromosomal Diversity within Gaura sect. Gaura (Onagraceae)

The proposed phylogeny of Gaura section Gaura is supported by experimental hy- bridization studies. The generally high average capsule-set of crosses between species of section Gaura (78%), compared with capsule-set averages for intersectional crosses (32%), agrees with a pattern of morphological similarity and geographical congruence in confirming the close relation- ships among these species. Higher average crossing success for G. longiflora and its derivatives, G. biennis and G. demareei, suggests the central importance of G. longiflora within the section. Measures of chromosomal diversity demonstrate the apparent conservation of G. longiflora chromosome end arrangements within both G. angustifolia and G. neomexicana, as well as supporting the hypothesis of a hybrid origin for G. demareei (G. lindheimeri x G. longiflora). Apparent levels of translocation diversity in natural populations depend upon the breeding system. Selfing tends to distribute structurally diverse arrangements among homozygous individuals. The extreme chromosomal di- versity of G. biennis sets it apart as a unique genetic system, as does its permanent translocation heterozygosity. Segregation distortion for chromosomal complexes from a ring of four chromo- somes is present in one individual of G. neomexicana. Speciation in section Gaura has not depended upon post-mating, reproductive isolating mechanisms. Factors that contribute to the maintenance of each species as a distinct natural entity are: geographical isolation due to ecological specialization and/or range expansion (G. angustifolia, G. biennis, G. lindheimeri); self-compatibility (G. angustifolia, G. biennis, G. neomexicana); displacement of seasonal (G. angustifolia, G. lindheimeri) and daily (G. demareei, G. lindheimeri) flowering times; and permanent translocation heterozygosity (G. biennis).

Functional significance of flexibility in proteins.

AbstractThe structural basis and the functional implications of large‐scale flexibility are discussed for three systems: trypsin–trypsinogen, immunoglobulins, and citrate synthase. The trypsin–trypsinogen system provides an example in which an order–disorder transition is used as a means to regulate enzymatic activity. Immunoglobulins demonstrate how flexibly linked domains may be used to allow the binding of ligands with diverse arrangements. In citrate synthase, domain motion forms an active site that is shielded from solvent. Analogous large‐scale flexibility has been observed in a number of other systems.

Functional significance of flexibility in proteins

Abstract

A cloned chicken DNA fragment includes two repeated DNA sequences with remarkably different genomic organizations.

The presence of both single copy sequences and repeated DNA sequences with a broad range of repetition frequency is a hallmark of the eukaryotic genome. The advent of recombinant DNA technology has made it possible to isolate cloned DNA fragments that encompass two or more DNA sequences with different repetition frequencies. This provides the first opportunity to investigate the structural relationships in the genome of DNA of the different repetition frequency classes in a thorough and systematic way. A cloned fragment of the chicken genome containing two different repeated DNA sequences has been used for this purpose. By hybridization of radioactive restriction fragments of the cloned DNA to filter-bound restriction fragments of total chicken DNA, it has been determined that the 5.5-kilobase inserted DNA in this clone contains a copy of each of two long, repeated DNA sequence elements with different repetition frequencies. These two repeated DNA sequences have very diverse arrangements in the chicken genome. The results establish that, in addition to the interspersion of single copy and repeated DNA sequences in the chicken genome, repeated DNA sequences with very different structural characteristics can be interspersed with one another. Thus, the chicken genome is a complex network of related sequences in which some members of a family of repeated DNA sequences are associated with other, nonhomologous repeated DNA sequences, while other members of the same family are flanked by single copy DNA.

Systematic anatomical characters of the leaves of Dimorphandra and Mora (Leguminosae: Caesalpinioideae)

A systematic study of the leaf anatomy of Dimorphandra and Mora showed that the leaf epidermis provides taxonomic characters for the separation of the two sections of Dimorphandra, Pocillum and Dimorphandra, viz. the structure of the guard cells and the degree of sunkenness of the stomata. The diverse arrangements of the neighbouring cells around the stomata do not provide a clear distinction between the sections: in section Pocillum the stomata are paracytic but in section Dimorphandra several types are found. Papillae of diverse shape, size and disposition are present in the different species of Dimorphandra, and with all these variations in the epidermal characters it is possible to establish series of increasing complexity in the two sections. Mora presents several characters which distinguish it from Dimorphandra (e.g. thickenings in the walls of the stomata and other epidermal cells, granular silica in the lumen of some epidermal cells, and sinuous anticlinal walls in the cells of both epidermides), thus supporting its recognition as a separate genus.

Concepts and Realities in International Political Organization

The heading under which these observations are brought together may well seem presumptuous. The request made to me was to deal with the subject: “collective security”—a phrase which I am hesitant to use, for it has come to cover the most diverse international political arrangements. The title adopted provides a convenient umbrella under which to bring together a number of general observations on the approach to the problem of maintaining peace.