Kinetochore reproduction in animal evolution: cell biological explanation of karyotypic fission theory

Chromosome Evolution of the Liolaemus monticola (Liolaemidae) Complex: Chromosomal and Molecular Aspects

Chromosomal rearrangements can directly influence population differentiation and speciation. The Liolaemus monticola complex in Chile is a unique model consisting of several chromosome races arranged in a latitudinal sequence of increasing karyotype complexity from south to north. Here, we compared chromosomal and mitochondrial cytochrome b data from 15 localities across the northern geographic distribution of L. monticola. We expanded the distribution of the previously described Multiple Fissions race (re-described as MF2), in the Coastal range between the Aconcagua River and the Petorca River, and described a new Multiple Fissions 1 (MF1) race in the Andean range. Both races present centric fissions in pairs 1 and 2, as well as a pericentric inversion in one fission product of pair 2 that changes the NOR position. Additionally, we detected a new chromosomal race north of the Petorca River, the Northern Modified 2 (NM2) race, which is polymorphic for novel centric fissions in pairs 3 and 4. Our results increase the number of chromosomal races in L. monticola to seven, suggesting a complex evolutionary history of chromosomal rearrangements, population isolation by barriers, and hybridization. These results show the relevant role of chromosome mutations in evolution, especially for highly speciose groups such as Liolaemus lizards.

From Empedocles to Symbiogenetics: Lynn Margulis's revolutionary influence on evolutionary biology

As a primary expositor of the work of Lynn Margulis collaborating with her over thirty years on over thirty books and forty articles, scientific and popular, I attempt here to summarize her unique and lasting influence on evolutionary biology. Describing life on Earth as the multi-billion-year evolution of microbial communities, from prokaryotes maintaining Earth's atmosphere away from thermodynamic equilibrium to all eukaryotes as polygenomic beings, Margulis's interdisciplinary work has deeply influenced multiple fields including systematics, theories of the evolution of metabolism, paleobiology, and biogeochemistry. Overturning the neo-Darwinist narrative that speciation almost always occurs by the gradual accumulation of random mutations, Margulis's work revives a discarded philosophical speculation of the pre-Socratic Empedocles, who suggested that Earth's early beings both merged and differentially reproduced (were naturally selected); a speculation that was rejected by Aristotle probably because it smacked of mythological chimeras that had no place in observational biology, and later by Charles Darwin, who mentioned Aristotle's rejection of Empedocles to show that he knew of but did not accept natural selection, thus helping lay his own claim to its own proper scientific presentation in a Victorian culture whose thinking of origins was dominated not by Greek mythology but Christian special creation. Margulis's curiosity-driven science, collaborative work ethic, status as a woman, embrace of novelty, philosophical stance, current status of her theories, and the proposal for a new science of symbiogenetics are among the topics examined.

The spindle assembly checkpoint and speciation

A mechanism is proposed by which speciation may occur without the need to postulate geographical isolation of the diverging populations. Closely related species that occupy overlapping or adjacent ecological niches often have an almost identical genome but differ by chromosomal rearrangements that result in reproductive isolation. The mitotic spindle assembly checkpoint normally functions to prevent gametes with non-identical karyotypes from forming viable zygotes. Unless gametes from two individuals happen to undergo the same chromosomal rearrangement at the same place and time, a most improbable situation, there has been no satisfactory explanation of how such rearrangements can propagate. Consideration of the dynamics of the spindle assembly checkpoint suggest that chromosomal fission or fusion events may occur that allow formation of viable heterozygotes between the rearranged and parental karyotypes, albeit with decreased fertility. Evolutionary dynamics calculations suggest that if the resulting heterozygous organisms have a selective advantage in an adjoining or overlapping ecological niche from that of the parental strain, despite the reproductive disadvantage of the population carrying the altered karyotype, it may accumulate sufficiently that homozygotes begin to emerge. At this point the reproductive disadvantage of the rearranged karyotype disappears, and a single population has been replaced by two populations that are partially reproductively isolated. This definition of species as populations that differ from other, closely related, species by karyotypic changes is consistent with the classical definition of a species as a population that is capable of interbreeding to produce fertile progeny. Even modest degrees of reproductive impairment of heterozygotes between two related populations may lead to speciation by this mechanism, and geographical isolation is not necessary for the process.

Chromosomal polymorphism in mammals: an evolutionary perspective

Although chromosome rearrangements (CRs) are central to studies of genome evolution, our understanding of the evolutionary consequences of the early stages of karyotypic differentiation (i.e. polymorphism), especially the non-meiotic impacts, is surprisingly limited. We review the available data on chromosomal polymorphisms in mammals so as to identify taxa that hold promise for developing a more comprehensive understanding of chromosomal change. In doing so, we address several key questions: (i) to what extent are mammalian karyotypes polymorphic, and what types of rearrangements are principally involved? (ii) Are some mammalian lineages more prone to chromosomal polymorphism than others? More specifically, do (karyotypically) polymorphic mammalian species belong to lineages that are also characterized by past, extensive karyotype repatterning? (iii) How long can chromosomal polymorphisms persist in mammals? We discuss the evolutionary implications of these questions and propose several research avenues that may shed light on the role of chromosome change in the diversification of mammalian populations and species.

Cytotaxonomy of unionid freshwater mussels (Unionoida, Unionidae) from northeastern Thailand with description of a new species

Morphological and chromosomal characteristics of a number of unionid freshwater mussels were studied from northeastern Thailand. Karyotypes of eight species from seven genera (Chamberlainia, Ensidens, Hyriopsis, Physunio, Pseudodon, Scabies and Trapezoideus) were examined. Six species possess 2n = 38 karyotypes, whereas Scabiescrispata and an unidentified Scabies sp. lack three small chromosome pairs, giving a diploid number of 32. Moreover, the karyotypes of the unidentified Scabies differ from Scabiescrispata as it exhibits a telocentric chromosome pair (6m + 7sm + 2st + 1t). Most of the conchological characters also differ between the two species – adult size, colour pattern, muscle scars, pseudocardinal and lateral teeth. The name Scabiessongkramensissp. n. is proposed for the unidentified species, and its description is included in this paper. Interestingly, seven species contain mostly bi-armed chromosomes, but only the mud-dweller in stagnant water, Ensidensingallsianus, contains predominantly five telocentric pairs. In addition, the marker chromosome characteristics of an unbalanced long arm, twisted centromere, a wider angle 180° arrangement, a twisted arm and telomeric end union reported in this study are described for the first time for unionid mussels.

The telomeric sync model of speciation: species-wide telomere erosion triggers cycles of transposon-mediated genomic rearrangements, which underlie the saltatory appearance of nonadaptive characters

Charles Darwin knew that the fossil record is not overwhelmingly supportive of genetic and phenotypic gradualism; therefore, he developed the core of his theory on the basis of breeding experiments. Here, I present evidence for the existence of a cell biological mechanism that strongly points to the almost forgotten European concept of saltatory evolution of nonadaptive characters, which is in perfect agreement with the gaps in the fossil record. The standard model of chromosomal evolution has always been handicapped by a paradox, namely, how speciation can occur by spontaneous chromosomal rearrangements that are known to decrease the fertility of heterozygotes in a population. However, the hallmark of almost all closely related species is a differing chromosome complement and therefore chromosomal rearrangements seem to be crucial for speciation. Telomeres, the caps of eukaryotic chromosomes, erode in somatic tissues during life, but have been thought to remain stable in the germline of a species. Recently, a large human study spanning three healthy generations clearly found a cumulative telomere effect, which is indicative of transgenerational telomere erosion in the human species. The telomeric sync model of speciation presented here is based on telomere erosion between generations, which leads to identical fusions of chromosomes and triggers a transposon-mediated genomic repatterning in the germline of many individuals of a species. The phenotypic outcome of the telomere-triggered transposon activity is the saltatory appearance of nonadaptive characters simultaneously in many individuals. Transgenerational telomere erosion is therefore the material basis of aging at the species level.

Centromeres were derived from telomeres during the evolution of the eukaryotic chromosome

The centromere is the DNA region of the eukaryotic chromosome that determines kinetochore formation and sister chromatid cohesion. Centromeres interact with spindle microtubules to ensure the segregation of chromatids during mitosis and of homologous chromosomes in meiosis. The origin of centromeres, therefore, is inseparable from the evolution of cytoskeletal components that distribute chromosomes to offspring cells. Although the origin of the nucleus has been debated, no explanation for the evolutionary appearance of centromeres is available. We propose an evolutionary scenario: The centromeres originated from telomeres. The breakage of the ancestral circular genophore activated the transposition of retroelements at DNA ends that allowed the formation of telomeres by a recombination-dependent replication mechanism. Afterward, the modification of the tubulin-based cytoskeleton that allowed specific subtelomeric repeats to be recognized as new cargo gave rise to the first centromere. This switch from actin-based genophore partition to a tubulin-based mechanism generated a transition period during which both types of cytoskeleton contributed to fidelity of chromosome segregation. During the transition, pseudodicentric chromosomes increased the tendency toward chromosomal breakage and instability. This instability generated multiple telocentric chromosomes that eventually evolved into metacentric or holocentric chromosomes.

Late-appearing pseudocentric fission event during chronic myeloid leukemia progression

Pseudocentric fission is a rare event consisting of the splitting of one functional centromere into two new products, of which only one can give rise to a functionally competent kinetochore. We report here a pseudocentric fission event within the D5Z2 alphoid subset disrupting the centromeric region of chromosome 5 in a case of chronic myeloid leukemia (CML) after treatment with imatinib and interferon. The breakage generated unequal partitioning of alpha-satellite sequences between the two fission products. One product was inserted within the long arm of chromosome 12 at band 14.3, becoming the only functional centromere of chromosome der(5). The other fission product was rearranged to form a sandwich-like dicentric--but functionally monocentric--chromosome der(6), made up of material from chromosomes 5, 12, and 6. The intercentric distance on der(6) was shown to be largely >20 Mb. To our knowledge, this is the first pseudocentric fission event described in CML. Moreover, our results confirm the susceptibility to breakage of the centromeric region of chromosome 5.

A de novo centric fission of chromosome 11 in a patient with recurrent miscarriages

We report on a de novo centric fission of chromosome 11 in a healthy female referred for chromosome analysis due to recurrent miscarriages. Both fission products were mitotically stable. This centric fission of chromosome 11 appears to have no clinical significance for this patient other than recurrent miscarriages.

Molecular distinction between true centric fission and pericentric duplication-fission

Centromere (centric) fission, also known as transverse or lateral centric misdivision, has been defined as the splitting of one functional centromere of a metacentric or submetacentric chromosome to produce two derivative centric chromosomes. It has been observed in a range of organisms and has been ascribed an important role in karyotype evolution; however, the underlying mechanisms remain unknown. We have investigated four cases of apparent centric fission in humans. Two cases show a missing chromosome 22 or 18 that is replaced by two centric ring products, a third case shows two chromosome-10-derived telocentric chromosomes, whereas a fourth case involves the formation of two chromosome-18-derived isochromosomes. In all four cases, results of gross cytogenetic and fluorescence in situ hybridisation analyses were consistent with a simple centric fission event. However, detailed molecular analyses provided evidence in support of centromere duplication as a predisposing mechanism for the observed chromosomal breakage in two of the cases. Results for the third case are consistent with direct centric fission not involving centromere pre-duplication as the likely mechanism. Insufficient material has precluded the further study of the fourth case. The data provide the first molecular evidence for centromere pre-duplication as a possible mechanism to explain the classically assumed simple "centric fission" events in clinical cytogenetics, karyotype evolution and speciation.

Integrative study on chromosome evolution of mammals, ants and wasps based on the minimum interaction theory

There is well-known evidence that in many eukaryotes, different species have different karyotypes (e.g. n=1-47 in ants and n=3-51 in mammals). Alternative (fusion and fission) hypotheses have been proposed to interpret this chromosomal diversity. Although the former has long been accepted, accumulating molecular genetics evidence seems to support the latter. We investigated this problem from a stochastic viewpoint using the Monte Carlo simulation method under the minimum interaction theory. We found that the results of simulations consistently interpreted the chromosomal diversity observed in mammals, ants and wasps, and concluded that chromosome evolution tends to evolve as a whole toward increasing chromosome numbers by centric fission. Accordingly, our results support the fission hypothesis. We discussed the process of chromosome evolution based on the latest theory of the molecular structure of chromosomes, and reconfirmed that the fission burst is the prime motive force in long-term chromosome evolution, and is effective in minimizing the genetic risks due to deleterious reciprocal translocations and in increasing the potential of genetic divergence. Centric fusion plays a biological role in eliminating heterochromatin (C-bands), but is only a local reverse flow in contrast to the previously held views.