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Animal cytology and evolution.
01 Jan 1977-
About: The article was published on 1977-01-01 and is currently open access. It has received 2447 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
3,307 citations
TL;DR: New estimates for the incidence of polyploidy in ferns and flowering plants are presented based on a simple model describing transitions between odd and even base chromosome numbers, and it is indicated that ploidy changes may represent from 2 to 4% of speciation events in flowering plants and 7% in f Ferns.
Abstract: ▪ Abstract Changes in ploidy occurred early in the diversification of some animal and plant lineages and represent an ongoing phenomenon in others. While the prevalence of polyploid lineages indicates that this phenomenon is a common and successful evolutionary transition, whether polyploidization itself has a significant effect on patterns and rates of diversification remains an open question. Here we review evidence for the creative role of polyploidy in evolution. We present new estimates for the incidence of polyploidy in ferns and flowering plants based on a simple model describing transitions between odd and even base chromosome numbers. These new estimates indicate that ploidy changes may represent from 2 to 4% of speciation events in flowering plants and 7% in ferns. Speciation via polyploidy is likely to be one of the more predominant modes of sympatric speciation in plants, owing to its potentially broad-scale effects on gene regulation and developmental processes, effects that can produce immed...
2,076 citations
TL;DR: Fisher’s model of stabilizing selection on multiple traits, under which reproductive isolation evolves as a side‐effect of adaptation in allopatry, confirms a priori arguments that while recombinant hybrids are less fit on average, some gene combinations may be fitter than the parents, even in the parental environment.
Abstract: Hybridization may influence evolution in a variety of ways. If hybrids are less fit, the geographical range of ecologically divergent populations may be limited, and prezygotic reproductive isolation may be reinforced. If some hybrid genotypes are fitter than one or both parents, at least in some environments, then hybridization could make a positive contribution. Single alleles that are at an advantage in the alternative environment and genetic background will introgress readily, although such introgression may be hard to detect. ‘Hybrid speciation’, in which fit combinations of alleles are established, is more problematic; its likelihood depends on how divergent populations meet, and on the structure of epistasis. These issues are illustrated using Fisher’s model of stabilizing selection on multiple traits, under which reproductive isolation evolves as a side-effect of adaptation in allopatry. This confirms a priori arguments that while recombinant hybrids are less fit on average, some gene combinations may be fitter than the parents, even in the parental environment. Fisher’s model does predict heterosis in diploid F1s, asymmetric incompatibility in reciprocal backcrosses, and (when dominance is included) Haldane’s Rule. However, heterosis arises only when traits are additive, whereas the latter two patterns require dominance. Moreover, because adaptation is via substitutions of small effect, Fisher’s model does not generate the strong effects of single chromosome regions often observed in species crosses.
1,436 citations
TL;DR: It is argued that rearrangements reduce gene flow more by suppressing recombination and extending the effects of linked isolation genes than by reducing fitness.
Abstract: Several authors have proposed that speciation frequently occurs when a population becomes fixed for one or more chromosomal rearrangements that reduce fitness when they are heterozygous This hypothesis has little theoretical support because mutations that cause a large reduction in fitness can be fixed through drift only in small, inbred populations Moreover, the effects of chromosomal rearrangements on fitness are unpredictable and vary significantly between plants and animals I argue that rearrangements reduce gene flow more by suppressing recombination and extending the effects of linked isolation genes than by reducing fitness This unorthodox perspective has significant implications for speciation models and for the outcomes of contact between neospecies and their progenitor(s)
1,237 citations