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PubMed2003

Genes that determine flower color: the role of regulatory changes in the evolution of phenotypic adaptations.

A central goal of evolutionary genetics is to trace the causal pathway between mutations at particular genes and adaptation at the phenotypic level. The proximate objective is to identify adaptations through the analysis of molecular sequence data from specific candidate genes or their regulatory elements. In this paper, we consider the molecular evolution of floral color in the morning glory genus (Ipomoea) as a model for relating molecular and phenotypic evolution. To begin, flower color variation usually conforms to simple Mendelian transmission, thus facilitating genetic and molecular analyses. Population genetic studies of flower color polymorphisms in the common morning glory (Ipomoea purpurea) have shown that some morphs are subject to complex patterns of selection. Striking differences in floral color and morphology are also associated with speciation in the genus Ipomoea. The molecular bases for these adaptive shifts can be dissected because the biosynthetic pathways that determine floral pigmentation are well understood and many of the genes of flavonoid biosynthesis have been isolated and extensively studied. We present a comparative analysis of the level of gene expression in Ipomoea for several key genes in flavonoid biosynthesis. Specifically we ask: how frequently are adaptive shifts in flower color phenotypes associated with changes in regulation of gene expression versus mutations in structural genes? The results of this study show that most species differences in this crucial phenotype are associated with changes in the regulation of gene expression.

Adaptation, Physiological↗
PubMed2004

Consistency of Nicotiana attenuata's herbivore- and jasmonate-induced transcriptional responses in the allotetraploid species Nicotiana quadrivalvis and Nicotiana clevelandii.

We examined the consistency of the native diploid Nicotiana attenuata (Na)'s herbivore-induced transcriptional changes in the two allotetraploid natives, Nicotiana clevelandii (Nc) and Nicotiana quadrivalvis (Nq), which are thought to be derived from hybridizations with an ancestral Na. An analysis of nuclear-encoded chloroplast-expressed Gln synthetase gene (ncpGS) sequences found strong similarity between Nc and Na and between N. trigonophylla and the two allopolyploids. All species were elicited with methyl jasmonate (MeJA), or were wounded and treated with either water, Manduca sexta oral secretions and regurgitant (R), or the two most abundant fatty acid amino acid conjugates (F) in R to simulate herbivory. The induced transcriptional responses in all three species were compared with a cDNA microarray enriched in Na genes. Na had the fastest transcriptional responses followed by Nc and then Nq. Na's R- and F-elicited responses were more similar to those from Nq, while the MeJA- or wound-elicited responses were more consistent in Nc. Treatment of wounds with the full cocktail of elicitors found in R elicits more complex responses than does treatment with F. The species differ in their elicited JA responses, and these differences are mirrored in the expression of oxylipin genes (LOX, HPL, AOS, and alpha-DOX) and downstream JA-elicited genes (TD). Elicitation decreases the expression of growth-related genes in all three species. We propose that this is a valuable system to examine the modification of complex, polygenic, adaptive responses during allopolyploid speciation.

Acetates↗
PubMed1979

Studies on DNA sequences in the Osmundaceae.

Phylogenetic relationships of Osmunda cinnamomea, O. claytoniana, and O. regalis were explored by means of DNA sequence comparisons. Hydroxyapatite thermal elution profiles of self-reassociated repetitive DNA fragments were very similar, indicating the absence of gross differences in the amount of recent amplification or addition of repetitive DNA in any of these three genomes. Interspecific DNA sequence comparisons showed, in contrast to our earlier interpretation, that repeated DNA sequences of O. claytoniana are nearly equally diverged from those of O. cinnamomea and O. regalis. Differences between repetitive sequences of the three species can be interpreted as reflecting amplification events which occurred subsequent to speciation. The data obtained suggest that the three Osmunda species most likely arose more or less simultaneously from a common ancestor. These findings were verified in experiments with tracer DNA preparations enriched for single copy sequences. On the basis of the hydridization data presented here and of the fossil record, the rate of single copy sequence divergence in the ferns is comparable to that in the primates, although slower than that observed in other animal taxa. From this first evaluation of rates of DNA evolution in plants it would seem that the rates for plants and animals are roughly comparable. The evidence suggests that species divergence is accompanied by further reiteration of preexisting repeat sequences. The rate of addition of repetitive sequences probably is slower in ferns than in angiosperms. This difference might be attributable to the much larger effective generation time in ferns.

Base Sequence↗
PubMed1997

Incipient species formation in salamanders of the Ensatina complex.

The Ensatina eschscholtzii complex of plethodontid salamanders, a well-known "ring species," is thought to illustrate stages in the speciation process. Early research, based on morphology and coloration, has been extended by the incorporation of studies of protein variation and mitochondrial DNA sequences. The new data show that the complex includes a number of geographically and genetically distinct components that are at or near the species level. The complex is old and apparently has undergone instances of range contraction, isolation, differentiation, and then expansion and secondary contact. While the hypothesis that speciation is retarded by gene flow around the ring is not supported by molecular data, the general biogeographical hypothesis is supported. There is evidence of a north to south range expansion along two axes, with secondary contact and completion of the ring in southern California. Current research targets regions once thought to show primary intergradation, but which molecular markers reveal to be zones of secondary contact. Here emphasis is on the subspecies E. e. xanthoptica, which is involved in four distinct secondary contacts in central California. There is evidence of renewed genetic interactions upon recontact, with greater genetic differentiation within xanthoptica than between it and some of the interacting populations. The complex presents a full array of intermediate conditions between well-marked species and geographically variable populations. Geographically differentiated segments represent a diversity of depths of time of isolation and admixture, reflecting the complicated geomorphological history of California. Ensatina illustrates the continuing difficulty in making taxonomic assignments in complexes studied during species formation.

Animals↗
PubMed2005

Isolation and mapping the chicken zona pellucida genes: an insight into the evolution of orthologous genes in different species.

The avian oocyte is surrounded by a specialized extracellular glycoproteinaceous matrix, the perivitelline membrane, which is equivalent to the zona pellucida (ZP) in mammals and the chorion in teleosts. A number of related ZP genes encode the proteins that make up this matrix. These proteins play an important role in the sperm/egg interaction and may be involved in speciation. The human genome is known to contain ZP1, ZP2, ZP3, and ZPB genes, while a ZPAX gene has also been identified in Xenopus. The rapid evolution of these genes has confused the nomenclature and thus orthologous relationships across species. In order to clarify these homologies, we have identified ZP1, ZP2, ZPC, ZPB, and ZPAX genes in the chicken and mapped them to chromosomes 5, 14, 10, 6, and 3, respectively, establishing conserved synteny with human and mouse. The amino acid sequences of these genes were compared to the orthologous genes in human, mouse, and Xenopus, and have given us an insight into the evolution of these genes in a variety of different species. The presence of the ZPAX gene in the chicken has highlighted a pattern of probable gene loss by deletion in mouse and gene inactivation by deletion, and base substitution in human.

Amino Acid Sequence↗
PubMed2006

Possible human impacts on adaptive radiation: beak size bimodality in Darwin's finches.

Adaptive radiation is facilitated by a rugged adaptive landscape, where fitness peaks correspond to trait values that enhance the use of distinct resources. Different species are thought to occupy the different peaks, with hybrids falling into low-fitness valleys between them. We hypothesize that human activities can smooth adaptive landscapes, increase hybrid fitness and hamper evolutionary diversification. We investigated this possibility by analysing beak size data for 1755 Geospiza fortis measured between 1964 and 2005 on the island of Santa Cruz, Galápagos. Some populations of this species can display a resource-based bimodality in beak size, which mirrors the greater beak size differences among species. We first show that an historically bimodal population at one site, Academy Bay, has lost this property in concert with a marked increase in local human population density. We next show that a nearby site with lower human impacts, El Garrapatero, currently manifests strong bimodality. This comparison suggests that bimodality can persist when human densities are low (Academy Bay in the past, El Garrapatero in the present), but not when they are high (Academy Bay in the present). Human activities may negatively impact diversification in 'young' adaptive radiations, perhaps by altering adaptive landscapes.

Adaptation, Physiological↗
PubMed1988

Comparison of cultural methods and DNA probe analyses for the detection of Actinobacillus actinomycetemcomitans, Bacteroides gingivalis, and Bacteroides intermedius in subgingival plaque samples.

The purpose of this study was to compare DNA probe analyses to cultural methods for detecting three periodontal pathogens, Actinobacillus actinomycetemcomitans, Bacteroides gingivalis, and Bacteroides intermedius, in human subgingival plaque. Subgingival sites from patients diagnosed as either healthy or showing evidence of gingivitis or juvenile or adult periodontitis were sampled using two paper points. The number of these pathogens from one paper point was determined using microbiologic media and speciated by biochemical tests. Results were then compared to bacterial numbers obtained from the other paper point using species-specific DNA probes. In 60 samples from the disease group, DNA probe analysis demonstrated 100% effectiveness in detecting A. actinomycetemcomitans and B. intermedius and 91% effectiveness in detecting B. gingivalis at culture positive levels (greater than or equal to 10(3) cells). In addition, probe assays frequently identified these pathogens in samples that were culture negative. Probe analysis revealed a better correlation between presence of a pathogen and clinical evidence of disease on an individual patient basis. In contrast, most samples taken from sites of healthy individuals showed undetectable levels of all three pathogens as determined by both techniques. These results suggest that DNA probe technology is at least equivalent and often superior to cultural methods for detecting A. actinomycetemcomitans, B. gingivalis, and B. intermedius in human subgingival plaque samples.

Actinobacillus↗
PubMed2003

Complex evolution of 7E olfactory receptor genes in segmental duplications.

Large segmental duplications (SDs) constitute at least 3.6% of the human genome and have increased its size, complexity, and diversity. SDs can mediate ectopic sequence exchange resulting in gross chromosomal rearrangements that could contribute to speciation and disease. We have identified and evaluated a subset of human SDs that harbor an 88-member subfamily of olfactory receptor (OR)-like genes called the 7Es. At least 92% of these genes appear to be pseudogenes when compared to other OR genes. The 7E-containing SDs (7E SDs) have duplicated to at least 35 regions of the genome via intra- and interchromosomal duplication events. In contrast to many human SDs, the 7E SDs are not biased towards pericentromeric or subtelomeric regions. We find evidence for gene conversion among 7E genes and larger sequence exchange between 7E SDs, supporting the hypothesis that long, highly similar stretches of DNA facilitate ectopic interactions. The complex structure and history of the 7E SDs necessitates extension of the current model of large-scale DNA duplication. Despite their appearance as pseudogenes, some 7E genes exhibit a signature of purifying selection, and at least one 7E gene is expressed.

Amino Acid Sequence↗
PubMed2000

The Drosophila serido speciation puzzle: putting new pieces together.

The D. serido superspecies is a complex mosaic of populations distributed over a vast part of South America and showing various degrees of genetical divergence. We have analyzed its chromosomal constitution in 16 new localities of southeastern and southern Brazil. Both the metaphase and salivary gland chromosomes show a sharp split of these populations in two groups. Four populations, fixed for inversion 2e8 and showing the type I karyotype, represent the southwestern limit of D. serido type B, which inhabits the Cerrado in central-western Brazil. The remaining populations are homozygous for 2x7, an inversion also fixed in the Caatinga populations of northeastern Brazil. However, their karyotype, in those populations analyzed, belong to a different type (V) from that of the Caatinga populations. Populations in this second group are polymorphic for five inversions on chromosome 2 plus another on chromosome 5 and show considerable interpopulation differentiation. The breakpoints of chromosome 2 inversions are described and the inversion loops of several heterokaryotypes are presented. Biogeographical information suggests that there are clear ecological differences between the two groups of populations as well as among the populations within the second group. Thepossible role of host plants in promoting the genetic divergence among the D. serido populations is discussed.

Animals↗
PubMed2000

Self-assembly of a diferrous triple-stranded helicate with bis(2,2'-bipyridine) ligands: thermodynamic and kinetic intermediates.

The protonation and iron(II) coordination properties of a bis(2,2'-bipyridine) ligand L were investigated in methanol. The protonated forms showed allosteric effects due to the flexibility of the strand. Speciation studies of the corresponding ferrous complexes were carried out as a function of pH and iron(II) concentrations. A combination of electrospray mass spectroscopy, potentiometry, and spectrophotometry allowed the determination in solution of three ferrous complexes, one mononuclear (L2Fe2+) and two dinuclear (L2Fe2(4+) and L3Fe2(4+)) species. Their structure was deduced from the metal spin state and confirmed by 1H NMR measurements and molecular modeling. The dissociation process of the triple-stranded diferrous helicate L3Fe2(4+) by OH- revealed two rate-limiting steps. The former leads to the formation of a monoferrous triple-stranded compound via a classical mechanism, which involves hydroxy-ferrous complexes. A similar process was observed in the latter step for the release of the ferrous cation from the mononuclear intermediate. Taking into account the structural, thermodynamic, and kinetic features provided by the present study, we could propose a self-assembling mechanism of the triple-stranded diferrous helicate.

2,2'-Dipyridyl↗
PubMed2000

Molecular cytogenetic analyses and phylogenetic studies in the Nicotiana section Tomentosae.

Phylogenetic schemes based on changing DNA sequence have made a major impact on our understanding of evolutionary relationships and significantly built on knowledge gained by morphological and anatomical studies. Here we present another approach to phylogeny, using fluorescent in situ hybridisation. The phylogenetic scheme presented is likely to be robust since it is derived from the chromosomal distribution of ten repetitive sequences with different functions and evolutionary constraints [GRS, HRS60, NTRS, the Arabidopsis-type telomere repeat (TTTAGGG)n, 18S-5.8S-26S ribosomal DNA (rDNA), 5S rDNA, and four classes of geminiviral-related DNA (GRD)]. The basic karyotypes of all the plant species investigated Nicotiana tomentosiformis, N. kawakamii, N. tomentosa, N. otophora, N. setchellii, N. glutinosa (all section Tomentosae), and N. tabacum (tobacco, section Genuinae) are similar (x=12) but the distribution of genic and non-genic repeats is quite variable, making the karyotypes distinct. We found sequence dispersal, and locus gain, amplification and loss, all within the regular framework of the basic genomic structure. We predict that the GRD classes of sequence integrated into an ancestral genome only once in the evolution of section Tomentosae and thereafter spread by vertical transmission and speciation into four species. Since GRD is similar to a transgenic construct that was inserted into the N. tabacum genome, its fate over evolutionary time is interesting in the context of the debate on genetically modified organisms and the escape of genes into the wild. Nicotiana tabacum is thought to be an allotetraploid between presumed progenitors of N. sylvestris (maternal, S-genome donor) and a member of section Tomentosae (T-genome donor). Of section Tomentosae, N. tomentosiformis has the most similar genome to the T genome of tobacco and is therefore the most likely paternal genome donor. It is known for N. tabacum that gene conversion has converted most 18S-5.8S-26S rDNA units of N. sylvestris origin into units of an N. tomentosiformis type. Clearly if such a phenomenon were widespread across the genome, genomic in situ hybridisation (GISH) to distinguish the S and T genomes would probably not work since conversion would tend to homogenise the genomes. The fact that GISH does work suggests a limited role for gene conversion in the evolution of N. tabacum.

DNA Probes↗
PubMed1982

Evolution of mouse immunoglobulin lambda genes.

The mouse has four C lambda and two V lambda genes. We have isolated Charon 4A clones that contain all six lambda genes from a BALB/c germ-line library. We present here the DNA sequences of the C lambda 2, C lambda 3, and C lambda 4 genes and also correct what are apparently errors in previous reports of C lambda 1 protein and DNA sequences. In addition, we have analyzed cloned DNAs by restriction mapping and electron microscopy to determine the relationships among the various lambda genes. By heteroduplex analysis, two gene clusters containing JC lambda 3--JC lambda 1 and JC lambda 2--JC lambda 4 show homology extending from the J regions 5' of C lambda 3/C lambda 2 to just 3' of C lambda 1/C lambda 4. Other than the region between the genes, very little homology exists in the C lambda flanking regions. In contrast, V lambda 1 and V lambda 2 genes show considerable homology extending into the 5' flanking regions. Large inverted repeats are found in the 5' flanking regions of V lambda 1 and C lambda 3, as well as in the 3' flanking regions of both C lambda gene clusters. DNA sequence divergences between the C lambda genes indicate that an ancestral JC lambda x--JC lambda g gene cluster arose at about the time of the first mammals by duplication of a primordial JC lambda gene. The data further suggest that the JC lambda x--JC lambda gene cluster duplicated after the speciation of mouse and man and subsequently diverged into the present day JC lambda 3--JC lambda 1 and JC lambda 2--JC lambda 4 gene clusters. C lambda 4, a pseudogene, became inactive at about the time of duplication of the ancestral JC lambda x--JC lambda y cluster. Comparison of DNA sequence divergence between the V lambda 1 and V lambda 2 genes demonstrates an anomaly. The percentage of amino acid replacement changes is approximately the same for V lambda 1/V lambda 2 as for C lambda 3/C lambda 2, implying that the ancestral V lambda gene was duplicated at the same time, and possibly together with, the JC lambda x--JC lambda y cluster. However, there are fewer silent changes than amino acid replacement changes between the V lambda 1/V lambda 2 genes, suggesting either that a selective pressure acted on the silent sites or that V lambda genes have only recently been duplicated. We also consider the possibility of a gene conversion event subsequent ot a more ancient duplication.

Animals↗
PubMed1995

Concerted evolution of the tandem array encoding primate U2 snRNA occurs in situ, without changing the cytological context of the RNU2 locus.

In primates, the tandemly repeated genes encoding U2 small nuclear RNA evolve concertedly, i.e. the sequence of the U2 repeat unit is essentially homogeneous within each species but differs somewhat between species. Using chromosome painting and the NGFR gene as an outside marker, we show that the U2 tandem array (RNU2) has remained at the same chromosomal locus (equivalent to human 17q21) through multiple speciation events over > 35 million years leading to the Old World monkey and hominoid lineages. The data suggest that the U2 tandem repeat, once established in the primate lineage, contained sequence elements favoring perpetuation and concerted evolution of the array in situ, despite a pericentric inversion in chimpanzee, a reciprocal translocation in gorilla and a paracentric inversion in orang utan. Comparison of the 11 kb U2 repeat unit found in baboon and other Old World monkeys with the 6 kb U2 repeat unit in humans and other hominids revealed that an ancestral U2 repeat unit was expanded by insertion of a 5 kb retrovirus bearing 1 kb long terminal repeats (LTRs). Subsequent excision of the provirus by homologous recombination between the LTRs generated a 6 kb U2 repeat unit containing a solo LTR. Remarkably, both junctions between the human U2 tandem array and flanking chromosomal DNA at 17q21 fall within the solo LTR sequence, suggesting a role for the LTR in the origin or maintenance of the primate U2 array.

Amino Acid Sequence↗
PubMed2006

Phylogenomics of several deer species revealed by comparative chromosome painting with Chinese muntjac paints.

A set of Chinese muntjac (Muntiacus reevesi) chromosome-specific paints has been hybridized onto the metaphases of sika deer (Cervus nippon, CNI, 2n = 66), red deer (Cervus elaphus, CEL, 2n = 62) and tufted deer (Elaphodus cephalophus, ECE, 2n = 47). Thirty-three homologous autosomal segments were detected in genomes of sika deer and red deer, while 31 autosomal homologous segments were delineated in genome of tufted deer. The Chinese muntjac chromosome X probe painted to the whole X chromosome, and the chromosome Y probe gave signals on the Y chromosome as well as distal region of the X chromosome of each species. Our results confirmed that exclusive Robertsonian translocations have contributed to the karyotypic evolution of sika deer and red deer. In addition to Robertsonian translocation, tandem fusions have played a more important role in the karyotypic evolution of tufted deer. Different types of chromosomal rearrangements have led to great differences in the genome organization between cervinae and muntiacinae species. Our analysis testified that six chromosomal fissions in the proposed 2n = 58 ancestral pecoran karyotype led to the formation of 2n = 70 ancestral cervid karyotype and the deer karyotypes is more derived compare with those of bovid species. Combining previous cytogenetic and molecular systematic studies, we analyzed the genome phylogeny for 11 cervid species.

Animals↗
PubMed2003

Evolution of bindin in the pantropical sea urchin Tripneustes: comparisons to bindin of other genera.

Bindin, a sea urchin sperm protein, mediates sperm-egg attachment and membrane fusion and is thus important in species recognition and speciation. Patterns of bindin variation differed among three genera that had been studied previously. In two genera of the superorder Camarodonta, Echinometra and Strongylocentrotus, both of which contain sympatric species, bindin is highly variable within and between species; a region of the molecule evolves at high rates under strong positive selection. In Arbacia, which belongs to the superorder Stirodonta and whose extant species are all allopatric, bindin variation is low, and there is no evidence of positive selection. We cloned and sequenced bindin from Tripneustes, a sea urchin that belongs to the Camarodonta but whose three species are found in different oceans. Worldwide sampling of bindin alleles shows that the bindin of Tripneustes (1) contains the highly conserved core characteristic of all other bindins characterized to date, (2) has an intron in the same position, and (3) has approximately the same length. Its structure is more like that of bindin from other camarodont sea urchins than to bindin from the stirodont ARBACIA: The resemblances to other camarodonts include a glycine-rich repeat structure upstream of the core and lack of a hydrophobic domain 3' of the core, a characteristic of Arbacia bindin. Yet the mode of evolution of Tripneustes bindin is more like that of Arbacia. Differences between bindins of the Caribbean Tripneustes ventricosus and the eastern Pacific T. depressus, separated for 3 my by the Isthmus of Panama, are limited to four amino acid changes and a single indel. There are no fixed amino acid differences or indels between T. depressus from the eastern Pacific and T. gratilla from the Indo-Pacific. Bindin of Tripneustes, like that of Arbacia, also shows no evidence of diversifying selection that would manifest itself in a higher proportion of amino acid replacements than of silent nucleotide substitutions. When the rate of intrageneric bindin divergence is standardized by dividing it by cytochrome oxidase I (COI) divergence, Tripneustes and Arbacia show a lower ratio of bindin to COI substitutions between the species of each genus than exists between the species of either Echinometra or Strongylocentrotus. Thus, mode of bindin evolution is not correlated with phylogenetic affinities or molecular structure, but rather with whether the species in a genus are allopatric or sympatric. For a molecule involved in gametic recognition, this would suggest a pattern of evolution via reinforcement. However, in bindin the process that gave rise to this pattern is not likely to have been selection to avoid hybridization, because there is no excess of amino acid replacements between species versus within species in the bindins of Echinometra and Strongylocentrotus, as would have been expected if specific recognition were the driving force in their evolution. We suggest instead that the pattern of reinforcement is a secondary effect of the ability of species with rapidly evolving bindins to coexist in sympatry.

Alleles↗
PubMed2004

Human centromere repositioning "in progress".

Centromere repositioning provides a potentially powerful evolutionary force for reproductive isolation and speciation, but the underlying mechanisms remain ill-defined. An attractive model is through the simultaneous inactivation of a normal centromere and the formation of a new centromere at a hitherto noncentromeric chromosomal location with minimal detrimental effect. We report a two-generation family in which the centromeric activity of one chromosome 4 has been relocated to a euchromatic site at 4q21.3 through the epigenetic formation of a neocentromere in otherwise cytogenetically normal and mitotically stable karyotypes. Strong epigenetic inactivation of the original centromere is suggested by retention of 1.3 megabases of centromeric alpha-satellite DNA, absence of detectable molecular alteration in chromosome 4-centromereproximal p- and q-arm sequences, and failure of the inactive centromere to be reactivated through extensive culturing or treatment with histone deacetylase inhibitor trichostatin A. The neocentromere binds functionally essential centromere proteins (CENP-A, CENP-C, CENP-E, CENP-I, BUB1, and HP1), although a moderate reduction in CENP-A binding and sister-chromatid cohesion compared with the typical centromeres suggests possible underlying structural/functional differences. The stable mitotic and meiotic transmissibility of this pseudodicentric-neocentric chromosome in healthy individuals and the ability of the neocentric activity to form in a euchromatic site in preference to a preexisting alphoid domain provide direct evidence for an inherent mechanism of human centromere repositioning and karyotype evolution "in progress." We discuss the wider implication of such a mechanism for meiotic drive and the evolution of primate and other species.

Autoantigens↗
PubMed1971

Heterochromatin, satellite DNA, and cell function. Structural DNA of eucaryotes may support and protect genes and aid in speciation.

With the assumption that a portion that comprises some 10 percent of the genomes in higher organisms cannot be without a raison d'être, an extensive review led us to conclude that a certain amount of constitutive heterochromatin is essential in multicellular organisms at two levels of organization, chromosomal and nuclear. At the chromosomal level, constitutive heterochromatin is present around vital areas within the chromosomes. Around the centromeres, for example, heterochromatin is believed to confer protection and strength to the centromeric chromatin. Around secondary constrictions, heterochromatic blocks may ensure against evolutionary change of ribosomal cistrons by decreasing the frequency of crossing-over in these cistrons in meiosis and absorbing the effects of mutagenic agents. During meiosis heterochromatin may aid in the initial alignment of chromosomes prior to synapsis and may facilitate speciation by allowing chromosomal rearrangement and providing, through the species specificity of its DNA, barriers against cross-fertilization. At the nuclear level of organization, constitutive heterochromatin may help maintain the proper spatial relationships necessary for the efficient operation of the cell through the stages of mitosis and meiosis. In the unicellular procaryotes, the presence of a small amount of genetic information in one chromosome obviates the need for constitutive heterochromatin and a nuclear membrane. At higher levels of organization, with an increase in the size of the genome and with evolution of cellular and sexual differentiation, the need for compartmentalization and structural components in the nucleus became imminent. The portion of the genome that was concerned with synthesis of ribosomal RNA was enlarged and localized in specific chromosomes, and the centromere became part of each chromosome when the mitotic spindle was developed in evolution. Concomitant with these changes in the genome, repetitive sequences in the form of constitutive heterochromatin appeared, probably as a result of large-scale duplication. The repetitive DNA's were kept through natural selection because of their importance in preserving these vital regions and in maintaining the structural and functional integrity of the nucleus. The association of satellite (or highly repetitive) DNA with constitutive heterochromatin is understandable, since it stresses the importance of the structural rather than transcriptional roles of these entities. Nuclear satellite DNA's have one property in common despite their species specificity, namely heterochromatization. In this sense the apparent species specificity of satellite DNA may be the result of natural selection for duplicated short polynucleotide segments that are nontranscriptional and can be utilized in specific structural roles.

Animals↗