https://www.cell.com/cell/pdf/0092-8674(87)90158-9.pdf

A genetic linkage map of the human genome

B chromosomes are extra chromosomes to the standard complement that occur in many organisms They can originate in a number of ways including derivation from autosomes and sex chromosomes in intraa

/pdf/b-chromosome-evolution-25cmhcd0kp.pdf

B-chromosome evolution.

Publisher Summary On a worldwide basis, plant breeding has been one of the most successful technologies developed in modern agriculture: its methods are opportunistic and adaptable to myriad production schemes, they require relatively inexpensive input, and their products have pervasive social benefits. DNA markers have become established as another tool for many phases of crop improvement, but the utility of this technology varies considerably with the application and context of the crop and culture. The world of DNA technology is changing rapidly, whereas plant breeding methodology has remained relatively stagnant. An awareness of genetic diversity and management of crop genetic resources have been important components of plant improvement programs. This chapter attempts to survey the status of selected applications of DNA fingerprinting for activities of common interest to plant breeding programs. There are a number of ways in which DNA markers could improve the management of plant genetic resources for the benefit of plant breeding programs and, ultimately, crop improvement. DNA markers have provided unprecedented opportunities for genetic resolution, and their use will expand, not as a panacea, but as a complement to the existing methods and their inherent limitations. The chapter presents data from several crops to illustrate the use of maps to reveal the features of crop genomes with implications for crop improvement. .

DNA Markers and Plant Breeding Programs

A map derived from restriction fragment length polymorphisms (RFLPs) in maize (Zea mays L.) is presented. The map was constructed in an immortalized Tx303 x CO159 F2 mapping population that allowed for an unlimited number of markers to be mapped and pooled F3 seed to be distributed to other laboratories. A total of 215 markers consisting of 159 genomic clones, 16 isozymes and 35 cloned genes of defined function have been placed on 10 chromosomes. An examination of segregation data has revealed several genomic regions with aberrant segregation ratios favoring either parent or the heterozygote. Mapping of cloned genes and isozymes that have been previously mapped by functional criteria has provided 29 points of alignment with the classical maize genetic map. Screening of all mapped RFLP probes against a collection of U.S. Corn Belt germplasm using EcoRI, HindIII and EcoRV has resulted in a set of 97 core markers being defined. The designation of a set of core markers allows the maize genome to be subdivided into a series of bins which serve as the backbone for maize genetic information and database boundaries. The merits and applications of core markers and bins are discussed.

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Development of a core RFLP map in maize using an immortalized F2 population.

To reproduce, higher plants utilize a unique multicellular microorganism: the male gametophyte, or pollen grain. The independent lifetime of flowering plant gametophytes is greatly abbreviated compared to that of gametophytes of more primitive plants, yet the angiosperm pollen grain must be able to survive at least briefly free from the sporophytic plant and perform a number of specialized functions before fertilization is accomplished. Although the pollen grain is a rather simple twoor threecelled organism, cytogenetic and mutagenesis experiments indicate that higher plants make a significant investment in genetic material devoted to gametophyte production (Carlson, 1977; Birchler and Schwartz, 1979; Kindiger et al., 1991). Molecular studies identifying antherand pollen-specific genes support the notion that the construction of a functional male gametophyte requires a rather large pool of such genes (Kamalay and Goldberg, 1980; Willing and Mascarenhas, 1984; McCormick, 1991). This is not so surprising given the specialized structures, mechanisms for rapid growth, and cell-cell communication systems during pollen-pistil interactions that have evolved to allow for both the survival of the gametophyte free of the sporophyte and the efficient delivery of sperm to the embryo sac. Studies of certain aspects of the molecular mechanisms of gametophyte development and function have progressed very rapidly. Examples of these include the identification of cis-acting elements determining tissue-specific gene expression in pollen and tapetal cells (Ursin et al., 1989; Koltunow et al., 1990; McCormick et al., 1991) and of mechanisms of self-incompatibility in pollen-pistil interactions (McClure et al., 1989, 1990; Thorsness et al., 1991). The intent of this article is to focus on a few less well studied but intriguing aspects of pollen development and function, with an emphasis on pollen development in maize. For more general reviews of microsporogenesis, the reader is referred to Giles and Prakash (1987) and Mascarenhas (1989). For recent reviews of gene expression during pollen development, see Mascarenhas (1990) and McCormick (1991). OVERVIEW OF POLLEN DEVELOPMENT

The remarkable biology of pollen.

The B chromosome in maize is small and highly heterochromatic. Its presence or absence in a plant has no apparent effect on the organism and the B is considered genetically inert. Studies with the maize B chromosome can be divided into two general categories: (1) analysis of the chromosome itself, including its origin, evolution, gene constitution, and DNA content, and (2) utilization of the chromosome in translocations to manipulate segments of standard (A) chromosomes. In the first category, an important finding has been that several genes exist on the B chromosome which help maintain its presence in populations through a system of non‐Mendelian inheritance. In the second type of research, a number of methods have been developed for manipulating A chromosome dosage with B‐A translocations. Segmental monosomics, trisomics, and tetrasomics have been constructed. Applications of the techniques are found mainly in cytological localization of genes and in construction of gene dosage series.

The B Chromosome of Maize

https://www.genetics.org/content/genetics/62/3/543.full.pdf

Factors affecting preferential fertilization in maize.

The Cytogenetics of Corn

TB-9Sb is a translocation between the B chromosome and chromosome 9 in maize. Certain deletions of B chromatin from the translocation cause a sharp decrease in B-9 transmission compared to the rate for standard TB-9Sb. The deletions remove components of a B chromosome genetic system that serves to suppress meiotic loss in the female. At least two distinct B-chromosome regions suppress meiotic loss: one on the B-9 and one on 9-B. The system operates by stabilizing univalent B-type chromosomes. It allows the univalents to migrate to one pole in meiosis, despite the absence of a pairing partner. The findings reported here are the first evidence for genetic control of meiotic loss by a B chromosome. However, it is proposed that the practice of suppressing meiotic loss is common to the B chromosomes of all species. The need to suppress meiotic loss results from the fact that B chromosomes are frequently unpaired in meiosis and subject to very high frequencies of loss. B chromosomes may utilize one or more of the following methods to suppress meiotic loss: (a) regular migration of univalent B's to one pole in meiosis, (b) enhanced recombination between B chromosomes and (c) mitotic nondisjunction.

/pdf/a-new-property-of-the-maize-b-chromosome-3oksoi8yk4.pdf

A new property of the maize B chromosome.

The B chromosome of maize has been used in a study of dicentric chromosomes. TB-9Sb is a translocation between the B and chromosome 9. The B-9 of TB-9Sb carries 60% of the short arm of 9. For construction of dicentrics, a modified B-9 chromosome was used, B-9-Dp9. It consists of the B-9 chromosome plus a duplicated 9S region attached to the distal end. In meiosis, fold-back pairing and crossing over in the duplicated region gives a chromatid-type dicentric B-9 that subsequently initiates a chromatid-type breakage-fusion-bridge cycle. In the male, it forms a single bridge in anaphase II of meiosis and at the first pollen mitosis. However, the cycle is interrupted by nondisjunction of the B centromere at the second pollen mitosis, which sends the B-9 dicentric to one pole and converts it from a chromatid dicentric to a chromosome dicentric. As expected, the new dicentric undergoes the chromosome-type breakage-fusion-bridge cycle and produces double bridges. A large number of plants with chromosome dicentrics were produced in this way. The presence of double bridges in the root cells of plants with a chromosome dicentric was studied during the first 10 wk of development. It was found that the number of plants and cells showing double bridges declined steadily over the 10-wk period. Several lines of evidence indicate that there was no specific developmental time for dicentric loss. "Healing" of broken chromosomes produced by dicentric breakage accounted for much of the dicentric loss. Healing produced a wide range of derived B-9 chromosomes, some large and some small. A group of minichromosomes found in these experiments probably represents the small end of the scale for B-9 derivatives.

https://www.genetics.org/content/genetics/153/3/1435.full.pdf

Wayne R. Carlson

Papers

The B Chromosome of Maize

Factors affecting preferential fertilization in maize.

The Cytogenetics of Corn

A new property of the maize B chromosome.

Time Course Study of the Chromosome-Type Breakage-Fusion-Bridge Cycle in Maize