Biomedical subjects

J P Camacho

Publications and source records attributed to J P Camacho.

15 recordsLinked to original sources

PubMed2001

Integration of a B chromosome into the A genome of a wasp.

B chromosomes are genome symbionts, the presence of which in many eukaryote species is explained, in most cases, by their violation of Mendelian rules, usually based on meiotic or mitotic instability, leading to their accumulation in the germ line (drive). However, B chromosome integration into the genome as a regular member of the chromosome set should imply the loss of drive. A possible way of bypassing this difficulty is to regularize meiosis when the B chromosome is frequent in the population, in order to yield gametes with one B chromosome. In diploid organisms, this task needs to be achieved in the two sexes, but in haplodiploids the problem simplifies to only the diploid sex. We have found, to the authors' knowledge, the first evidence of a B chromosome that is regularizing its meiotic behaviour and limiting its number to one B chromosome per haploid genome, the same dosage as the standard (A) chromosomes, in the solitary wasp Trypoxylon albitarse. It suggests a possible mechanism for B chromosome integration as a regular member of the chromosome complement.

Animals↗

PubMed2000

B-chromosome evolution.

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 intra- and interspecies crosses. Their subsequent molecular evolution resembles that of univalent sex chromosomes, which involves gene silencing, heterochromatinization and the accumulation of repetitive DNA and transposons. B-chromosome frequencies in populations result from a balance between their transmission rates and their effects on host fitness. Their long-term evolution is considered to be the outcome of selection on the host genome to eliminate B chromosomes or suppress their effects and on the B chromosome's ability to escape through the generation of new variants. Because B chromosomes interact with the standard chromosomes, they can play an important role in genome evolution and may be useful for studying molecular evolutionary processes.

Animals↗

PubMed2000

Fitness effect analysis of a heterochromatic supernumerary segment in the grasshopper Eyprepocnemis plorans.

Several components of fitness were analysed in relation to the presence of a supernumerary chromosome segment (SCS) in two natural populations of the grasshopper Eyprepocnemis plorans, including clutch size, egg fertility, egg and embryo productivity and survivability from embryo to adult, and SCS transmission through males. The results have shown the absence of a significant relationship between SCS presence and these fitness components, with the single exception of egg fertility which decreases significantly in SCS females with mating shortage. This fertility decrease is thus expected to be relevant for the population dynamics of the SCS only in low-density populations, those in which it is difficult for females to find a male to copulate with before each egg-batch is ready to be laid. The analysis of the SCS transmission through males showed no significant differences between expected and observed SCS frequencies. The SCS polymorphism seems to be at a status close to neutrality in respect to fitness, but its slight disadvantage in transmission through females carrying B chromosomes predicts that the polymorphism should tend to disappear, unless SCS recurrent amplification, or another undiscovered force, counteracts this tendency.

Analysis of Variance↗

PubMed2000

Frequency increase and mitotic stabilization of a B chromosome in the fish Prochilodus lineatus.

Six populations of the fish Prochilodus lineatus were analysed for B chromosome frequency. A study of spermatogenesis revealed the absence of B accumulation during the stages analysed. In one of the populations, from the Mogi-Guaçu river where samples have been analysed over a ten-year period, B chromosome frequency doubled between 1979-80 and 1987 89, whereas no additional changes were noticed in samples collected in 1991-92. The analysis of B chromosome mitotic instability, manifested by intraindividual variation in B chromosome number, indicated a very significant decrease during this time period. This suggests that, in the 1980s, this population was in the final stage of B chromosome invasion, and that there was a possible causal relationship between B mitotic instability and the accumulation mechanism that caused its frequency increase. Mitotic stabilization might thus be a way by which a mitotically unstable B chromosome may become neutralized.

Animals↗

PubMed2000

Structural and functional evidence that a B chromosome in the characid fish Astyanax scabripinnis is an isochromosome.

Astyanax scabripinnis possesses a widespread polymorphism for metacentric B chromosomes as large as the largest chromosome pair in the A complement. On the basis of C-banding pattern, it was hypothesized that these B chromosomes are isochromosomes that have arisen by means of centromere misdivision and chromatid nondisjunction. In the present paper we test this hypothesis by analysing (i) the localization of a repetitive DNA sequence on both B chromosome arms, and (ii) synaptonemal complex formation, in order to test the functional homology of both arms. Genomic DNA digested with KpnI and analysed by gel electrophoresis showed fragments in a ladder-like pattern typical of tandemly repetitive DNA. These fragments were cloned and their tandem organization in the genome was confirmed. A 51-bp long consensus sequence, which was AT-rich (59%) and contained a variable region and two imperfect reverse sequences, was obtained. Fluorescence in situ hybridization (FISH) localized this repetitive DNA into noncentromeric constitutive heterochromatin which encompasses the terminal region of some acrocentric chromosomes, the NOR region, and interstitial polymorphic heterochromatin in chromosome 24. Most remarkably, tandem repeats were almost symmetrically placed in the two arms of the B chromosome, with the exception of two additional small clusters proximally located on the slightly longer arm. Synaptonemal complex (SC) analysis showed 26 completely paired SCs in males with 1B. The ring configuration of the B univalent persisting until metaphase I suggests that the two arms formed chiasmata. All these data provided strong support for the hypothesis that the B chromosome is an isochromosome.

Animals↗

PubMed2000

Altitudinal variation for B chromosome frequency in the characid fish Astyanax scabripinnis.

The analysis of three populations of the characid fish Astyanax scabripinnis located at different altitudes along the same stream has revealed the presence of a macro B chromosome in two high-altitude populations (1800 m and 1920 m) but its absence from a low-altitude population (700 m). Because the stream flows through very rugged mountains, with numerous falls ensuring that any gene flow occurs downstream only, the absence of B chromosomes from the low-altitude populations is best interpreted in the light of the parasitic theory of B chromosome evolution. Under this theory, we would expect B chromosomes to be more frequent where environmental conditions are more favourable for the species, because Bs are best tolerated there. The widespread presence of these B chromosomes in numerous Brazilian river headwaters, which are the preferred habitat for this species, support this possibility.

Altitude↗

PubMed2000

B chromosomes in two fish species, genus Rhamdia (Siluriformes, Pimelodidae).

Specimens belonging to two fish species genus Rhamdia were cytogenetically analysed from seven localities in Brazil and Argentina. In addition to the 58 chromosomes of the basic karyotype, one to five metacentric B chromosomes were observed carrying conspicuous heterochromatic blocks on the distal regions of both chromosome arms. These B chromosomes are mitotically stable and, in the two best-sampled populations in R. hilarii (Lobo and 29 reservoirs), they showed frequency distributions fitting a binomial distribution, though Bs were more frequent in the latter. The presence of B chromosomes with the same appearance in R. quelen suggests an ancient origin for these B chromosomes, presumably prior to speciation from a common ancestor.

Animals↗

PubMed1999

Unusually high amount of inactive ribosomal DNA in the grasshopper Stauroderus scalaris.

Fluorescence in-situ hybridization (FISH) was employed to determine the chromosomal location of the ribosomal DNA cistrons in spermatocytes of two populations of the grasshopper Stauroderus scalaris. The results showed that paracentromeric C-bands, which in this species constitute about 50% of the total chromatin, contain substantial amounts of rDNA in all chromosomes. However, silver impregnation showed the presence of a single active nucleolus organizing region (NOR) in chromosome 3 of primary spermatocytes, indicating an extremely high amount of silent rDNA across the whole genome of this species in the two geographically distant populations analysed. The significance of such an unusual phenomenon is discussed.

Animals↗

PubMed1997

Geographical distribution of B chromosomes in the grasshopper Eyprepocnemis plorans, along a river basin, is mainly shaped by non-selective historical events.

The analysis of 19 populations of the grasshopper Eyprepocnemis plorans, collected along four rivers belonging to the Segura basin (Mundo, Benamor, Taibilla and Segura itself), has shown that the presence of B chromosomes ends abruptly in each river, coinciding with the existence of a narrow pass in which this grasshopper cannot live because of the absence of the appropriate habitats. The existence of a broad inland region lacking grasshoppers with B chromosomes suggests that B chromosomes arose after the first colonization of the Iberian Peninsula by E. plorans specimens from North African populations. The B chromosome seems to have spread upstream along each of these four rivers until reaching geographical barriers that have impeded its advance and thus have preserved the non-B chromosome region. The available evidence indicates that the observed geographical distribution of the B chromosome polymorphism in this zone was shaped mainly by historical non-selective events.

Altitude↗

PubMed1996

Achiasmate segregation of X and B univalents in males of the grasshopper Eyprepocnemis plorans is independent of previous association.

B chromosomes proved to be more frequent in males than females of the grasshopper Eyprepocnemis plorans collected from the population in Jete (Granada, Spain) in 1992. The meiotic behaviour of the X and B univalents was analysed in a high number of 1B males collected from this population in 1991 and 1992, and in males from another population (Salobreña, Granada, Spain) for comparison. These two chromosomes showed a significant tendency to migrate to opposite poles in the Jete population, during the 2 years analysed, but separated randomly in the Salobreña population. Thus, sex differences in the B frequency in Jete seemed to be due to the non-random X-B segregation during male meiosis. The analysis of association patterns between the two univalents over several stages of the first meiotic division indicated a heterochromatic affinity rather than association by chiasmata because most X-B associations had resolved by metaphase I. The X and B chromosomes share two different DNA sequences, so that some associations during prophase I undoubtedly involve homologous DNA sequences. The frequency with which X and B migrated to opposite poles at anaphase I in Jete, however, did not show any significant dependence on previous association at zygotene, diplotene or metaphase I.

Anaphase↗

PubMed1996

Sex-ratio distortion associated with the presence of a B chromosome in Astyanax scabripinnis (Teleostei, Characidae).

Cytogenetic studies were conducted on 154 specimens of Astyanax scabripinnis collected at three localities in the Campos do Jordão region (State of São Paulo, Brazil). The C-banding pattern suggested that the metacentric B chromosome found in most of the specimens is an isochromosome derived from chromosome 24, the only chromosome in the standard complement that carries interstitial C-bands similar to those present in each arm of the B chromosome. The sex ratio was biased toward females in the Córrego das Pedras and Ribeirão do Casquilho streams and toward males in the Ribeirão das Perdizes stream. In all three populations analyzed, the B chromosome was more frequent in females than in males. In the most exhaustively sampled population (those from Córrego das Pedras), there was a highly significant association between B-chromosome frequency and sex-ratio distortion, with a disproportionately high number of males without B chromosomes and females with one B chromosome.

Alleles↗

PubMed1995

Cloning and sequence analysis of an extremely homogeneous tandemly repeated DNA in the grasshopper Eyprepocnemis plorans.

Digestion of total nuclear DNA of the grasshopper Eyprepocnemis plorans with seven different restriction endonucleases (REs), and subsequent agarose gel electrophoresis, has shown the presence of highly repetitive DNA yielding the typical ladder-like banding pattern. The most clear pattern was produced by DraI, the monomer being some 180 bp. This repeat unit was subsequently cloned and sequenced. Bidirectional sequencing of five randomly chosen clones showed exactly the same nucleotides in all 180 positions. The possible explanations for such an extreme homogeneity of this tandem repeat are discussed in the light of current hypotheses on repetitive DNA function and molecular drive mechanisms.

Animals↗

PubMed1994

Possible origin of a B chromosome deduced from its DNA composition using double FISH technique.

Double fluorescent in situ hybridization (FISH) with two DNA probes (a 180 bp tandemly repeated DNA and ribosomal DNA) was performed in embryo cells of the grasshopper Eyprepocnemis plorans. Repetitive DNA was present in most standard chromosomes (excepting 7, 8 and 10) and in the proximal two-thirds of the B chromosome, which was its major location in the complement. Ribosomal DNA was present distally on the B, and in the active nucleolar organizer regions (NORs) of the X, 9, 10 and 11 chromosomes. A small number of rRNA gene clusters was also observed in the pericentromeric regions of chromosomes 1-8. The double FISH technique showed that the B chromosome (B2 type) is mainly composed of a 180 bp tandem repeat and ribosomal DNA, the minute short arm being the only region that does not hybridize with them. The location and order of the centromere and both the DNA sequences on the B chromosome coincide only with those in the X chromosome, indicating that the B most probably derives from the X.

Animals↗

PubMed1992

Male and female segregation distortion for heterochromatic supernumerary segments on the S8 chromosome of the grasshopper Chorthippus jacobsi.

The mode of inheritance of supernumerary segments located on three different chromosome pairs was investigated in controlled crosses with specimens of the grasshopper Chorthippus jacobsi. While extra segments located on chromosomes M5 and M6 showed Mendelian inheritance, that on S8 did not. Thus, the two supernumerary heterochromatic chromosome segments located distally on the S8 chromosome accumulated through non-Mendelian transmission through both sexes. The observed transmission patterns may be explained by gametic selection for spermatozoa carrying segmented S8 chromosomes, in addition to meiotic drive for segmented S8 chromosomes in heterozygous females. The significance of these findings for the maintenance of these polymorphisms in natural populations is discussed.

Animals↗

PubMed1991

Role of C-heterochromatin in variation of nuclear DNA amount in the genus Chorthippus (Orthoptera, Acrididae).

Nuclear DNA amounts of nine species of grasshopper genus Chorthippus were determined in populations of southern Spain. There were significant differences between species, and between populations within certain species, for this character. Measurements of chromosomes in C-banded mitotic metaphase cells enabled the relative amounts of positively and negatively C-banded chromatin to be ascertained. This demonstrated that variations observed in nuclear DNA amount are due to changes in both types of chromatin but those in C-heterochromatin are much more important.

Animals↗