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Recent advances in cytology
01 Jan 1932-
About: The article was published on 1932-01-01 and is currently open access. It has received 1048 citations till now. The article focuses on the topics: Cytology.
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TL;DR: At the present time, with the immense increase in research activity in mammalian cytology, the terminology of the centromeric position has become burdened by much obscurity and confusion.
Abstract: N the inorphologic identification of chromosomes, the location of the I centromere is the most useful landmark, and one which is characterized by great constancy. It would seem that not much could be added to the definitions by E. B. WILSON (1928) of the locations on the chromosome of the centrornere or, in the terminology of that time, the spindle attachment: “Attachment of the chromosome to the spindle is commonly limited to a small area, and is of two general types, namely: (1) terminnl or telomitic and (2) non-ferminal or atelomitic, being in the former case at one end, and in the latter at some other point or points. Non-terminal attachment may be at the middle point (median) or at an intermediate point (submedian, sub-terminal). All gradations exist between these various cases;” (I.c., p. 130-131). In the acconipanying picture (l.c., Fig. 56, p. 132), here reprinted as Fig. l., the four locations of median, submedian, subterminal and terminal are represented, and, in addition, “lateral”, which corresponds to the modern term “diffuse centromere”. Nevertheless, at the present time, with the immense increase in research activity in mammalian cytology, the terminology of the centromeric position has become burdened by much obscurity and confusion. One cause of confusion is that different authors, and even the same author on different occasions, have used the terms median, submedian etc. with great amplitude, and it is often difficult to know in a specific case what each term signifies. Another cause of confusion is that a set of terms for chromosomes with specific centromeric positions, such as metacentric, acrocentric, telocentric, have come into wide usage without being clearly defined in relation to the positional terms median, submedian, subterminal and terminal. During the spring of 1963 the present writers exchanged epistolary
5,352 citations
TL;DR: The results indicate that the triploid bridge pathway can contribute significantly to autopolyploids formation regardless of the mating system, and to allopolyploid formation in outcrossing taxa.
Abstract: Polyploidy is widely acknowledged as a major mechanism of adaptation and speciation in plants. The stages in polyploid evolution include frequent fertility bottlenecks and infrequent events such as gametic nonreduction and interspecific hybridization, yet little is known about how these and other factors influence overall rates of polyploid formation. Here we review the literature regarding polyploid origins, and quantify parameter values for each of the steps involved in the principal pathways. In contrast to the common claim that triploids are sterile, our results indicate that the triploid bridge pathway can contribute significantly to autopolyploid formation regardless of the mating system, and to allopolyploid formation in outcrossing taxa. We estimate that the total rate of autotetraploid formation is of the same order as the genic mutation rate (10 i5 ), and that a high frequency of interspecific hybridization (0.2% for selfing taxa, 2.7% for outcrossing taxa) is required for the rate of tetraploid formation via allopolyploidy to equal that by autopolyploidy. We conclude that the rate of autopolyploid formation may often be higher than the rate of allopolyploid formation. Further progress toward understanding polyploid origins requires studies in natural populations that quantify: (a) the frequency of unreduced gametes, (b) the effectiveness of triploid bridge pathways, and (c) the rates of interspecific hybridization.
1,715 citations
Cites background from "Recent advances in cytology"
...However, the possession of an unmatched complement of chromosomes leads to the formation of multivalents and univalents during pairing, and subsequent irregularities during disjunction can create varied chromosome assortments (33)....
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TL;DR: The time is therefore right for a review of the role of hybridization in plant speciation, demonstrating that perhaps all angiosperms have likely undergone at least one round of polyploidization and that hybridization has been an important force in generating angiosperm species diversity.
Abstract: The importance of hybridization in plant speciation and evolution has been debated for decades, with opposing views of hybridization as either a creative evolutionary force or evolutionary noise. Hybrid speciation may occur at either the homoploid (i.e., between two species of the same ploidy) or the polyploid level, each with its attendant genetic and evolutionary consequences. Whereas allopolyploidy (i.e., resulting from hybridization and genome doubling) has long been recognized as an important mode of plant speciation, the implications of genome duplication have typically not been taken into account in most fields of plant biology. Recent developments in genomics are revolutionizing our views of angiosperm genomes, demonstrating that perhaps all angiosperms have likely undergone at least one round of polyploidization and that hybridization has been an important force in generating angiosperm species diversity. Hybridization and polyploid formation continue to generate species diversity, with several n...
1,106 citations
Cites background or methods from "Recent advances in cytology"
...Inference of ploidy in angiosperms from chromosome numbers and hypothesized breaks between diploid and polyploid base numbers yielded estimates of 30–35% (193), to nearly 50% (40, 62, 66, 135), to as high as 70–80% (62, 102)....
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...In fact, a high degree of differentiation between diploid parents was considered much more likely to lead to polyploidization than to stabilization of a homoploid hybrid (34, 40)....
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...This approach was later employed by other early workers (34, 40, 66, 135)....
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TL;DR: The fertility of early generation polyploids increases rapidly, owing largely to selection against meiotic configurations that generate unbala..., casting doubt on the paradigm that allopolyploids should be more frequent due to their inherent fertility.
Abstract: ▪ Abstract Here we review the biology of early generation neopolyploids and discuss the profound changes that accompany their formation. Newly formed auto- and allopolyploids exhibit considerable meiotic complexity, including multivalent pairing, multisomic inheritance, and the production of unbalanced gametes. The cytogenetic behavior of allopolyploids and autopolyploids differ statistically, but are more similar than commonly believed. The progeny of neopolyploids include a high frequency of aneuploids, pseudoeuploids and homeologue-recombinant genotypes that may contribute to the phenotypic variability observed in early generation polyploids. We find no evidence to support the traditional view that autopolyploids possess lower fertility than allopolyploids, casting doubt on the paradigm that allopolyploids should be more frequent due to their inherent fertility. The fertility of early generation polyploids increases rapidly, owing largely to selection against meiotic configurations that generate unbala...
884 citations
TL;DR: Findings suggest that Mlh1 is involved in DNA mismatch repair and meiotic crossing over in mice deficient in another mismatch repair gene, M lh1.
Abstract: Mice that are deficient in either the Pms2 or Msh2 DNA mismatch repair genes have microsatellite instability and a predisposition to tumours. Interestingly, Pms2–deficient males display sterility associated with abnormal chromosome pairing in meiosis. Here mice deficient in another mismatch repair gene, Mlh1, possess not only microsatellite instability but are also infertile (both males and females). Mlh 1 –deficient spermatocytes exhibit high levels of prematurely separated chromosomes and arrest in first division meiosis. We also show that Mlh1 appears to localize to sites of crossing over on meiotic chromosomes/Together these findings suggest that Mlh1 is involved in DNA mismatch repair and meiotic crossing over.
794 citations