2000
 

J. P. M. Camacho, T. F. Sharbel, L. W. Beukeboom B chromosome evolution Phil. Trans. R. Soc. Lond. B 355: 163-178. (2000)

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 Bs or suppress their effects, and on the Bs 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.

 
 

C. A. Mestriner, P. M. Galetti Jr, S. R. Valentini, I. R. G. Ruiz, L. D. S. Abel, O. Moreira-Filho, J. P. M. Camacho Structural and functional evidence that a B chromosome in the characid fish Astyanax scabripinnis is an isochromosome Heredity 85: 1-9. (2000)

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  hypothetized that these B chromosomes are isochromosomes that have arisen by means of centromere misdivision and chromatid non-disjunction. In the present paper we test this hypothesis by analyzing 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 tamdem 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 non-centromeric constitutive heterochromatin which encompasses the terminal region of some acrocentric chromosomes, the NOR region, and interstitial polymorphic heterochromatin at 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 sugests that the two arms formed chiasmata. All these data provided strong support for the hypothesis that the B chromosome is an isochromosome.

 
 

F. Perfectti, J. Cabrero, M. D. López-León, E. Muñoz, M. C. Pardo, J. P. M. Camacho Fitness effect analysis of a heterochromatic supernumerary segment in the grasshopper Eyprepocnemis plorans Chromosome Research 8: 425-433. (2000)

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 not 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 other undiscovered force, counteracts this tendency

 

D. M. Néo, O. Moreira-Filho, J. P. M. Camacho Altitudinal variation for B chromosome frequency in the characid fish Astyanax scabripinnis Heredity 85: 136-141. (2000)

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). Since 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.

 

Z. I. Cavallaro, L. A. C. Bertollo, F. Perfectti, J. P. M. Camacho Frequency increase and mitotic stabilization of a B chromosome in the fish Prochilodus lineatus Chromosome Research 8: 627-634. (2000)

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, Mogi-Guaçu river, where samples have been analysed over a ten year period, B chromosome frequency was 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 intra-individual variation in B chromosome number, indicated a very significant decrease during this time period. This suggests that, in the 1980’s, 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 stabilisation might thus be a way by which a mitotically unstable B chromosome may become neutralised.

 

A. S. Fenocchio, L. A. C. Bertollo, C. S. Takahashi, J. P. M. Camacho B chromosomes in two fish species, genus Rhamdia (Siluriformes, Pimelodidae) Folia Biologica (Kraków) 48: 105-109. (2000)

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, but 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.

1999
 

F. Perfectti, J.P.M. Camacho Analysis of genotypic differences in developmental stability in Annona cherimola Evolution 53: 1396-1405 (1999)

The genetic basis of developmental stability, measured as asymmetry (fluctuating asymmetry in leaves), was analyzed in leaves and flowers of cherimoya (Annona cherimola Mill) and atemoya (A. cherimola x A. squamosa). The individuals analyzed belonged to a controlled collection of cultivars (clones) that had previously been characterized by means of isozymes. We used a nested design to analyze the differences in asymmetry at several sampling levels: individual leaves and flowers, individual trees, and genotypes. The clonal repeatability of developmental stability was not significantly different from zero, thus suggesting the absence of heritability of the asymmetry for leaves and flowers under these environmental conditions. No relationship between asymmetry and individual heterozygosity was found, but leaf fluctuating asymmetry was significantly related to particular isozymic genes. Petal and leaf size showed a phenotypically plastic response to the exposure zone of the tree (mainly due to light). Leaf fluctuating asymmetry also showed such a plastic response. No significant correlation was found between asymmetry and any pomological characters (some of these being fitness-related). Finally, the hybrid species (atemoya) did not show larger developmental instability than did the parental species (cherimoya). All these data show that cherimoya asymmetry reveals the random nature of developmental noise, with developmental stability for leaves being possibly related to specific chromosome regions, but with weak evidence for genotypic differences in developmental stability.

 

Bakkali, M., Cabrero, J., López-León, M. D., Perfectti, F., Camacho, J. P. M. The B chromosome polymorphism of the grasshopper Eyprepocnemis plorans in North Africa. I. B variants and frequency  Heredity 83: 428-434 (1999)

Polymorphism for B-chromosomes has been detected in all nine populations of the grasshopper Eyprepocnemis plorans ssp. plorans sampled in Morocco.  The most frequent B chromosome in all populations showed a C-banding pattern and size similar to those of the B1 variant found in the Iberian Peninsula. In addition, other B chromosome variants (B1iso1, B1iso2, B1d1, B1di1, B3 and B1dd1) were discovered in these populations, although at a very low frequency. No significant differences in B chromosome frequency were found either in the nine populations or, for some of them, in up to three consecutive years. These results are discussed in the light of current hypotheses on the evolution of this B chromosome polymorphism in the Iberian Peninsula.

 

J. Cabrero, M. D. López-León, M. Bakkali, J. P. M. Camacho Common origin of B chromosome variants in the grasshopper Eyprepocnemis plorans Heredity 83: 435-439 (1999)

Nine B chromosome variants from seven different populations collected at four localities in Spain and three others in Morocco, have shown to be mainly composed of two DNA sequences, i.e. a 180 bp tandem repeat and ribosomal DNA. B types, however, differ in the relative amounts of the two sequences. The most widespread one (B1) bears about the same amount of rDNA and 180 bp repeat, but three other variants that have reached a polymorphism by replacing B1 in smaller areas (B2, B5 and B24) carry a conspicuously larger amount of the 180 bp repeat. In Morocco, the most widespread B variant is also B1, and a rare variant that appeared in a single individual is also built with the two same DNA sequences. All these data point to a common origin for these B chromosomes, with B1 probably being the original one. The origin of the different B types and the possible relationship of the relative amount of 180 bp DNA repeat with B drive, are discussed.

 

J. Cabrero, L.H. Shapiro, J.P.M. Camacho Male sterility in interspecific meadow katydid hybrids Hereditas 131: 79-82 (1999)

Spermatogenesis was cytologically analyzed in males of the conocephaline katydids Orchelimum nigripes and O. pulchellum, their hybrids obtained in the laboratory, and males from intermediate populations belonging to a broad hybrid zone. The results showed a very high similarity between the two species for the cytogenetic markers analyzed (C-heterochromatin content and location of nucleolus organizer regions), but artificial hybrids appeared to be completely sterile due to a complete absence of meiosis and spermatozoa in their reduced testes

 

M.D. López-León, J. Cabrero and J.P.M. Camacho Unusually high amount of inactive ribosomal DNA in the grasshopper Stauroderus scalaris Chromosome Res. 7: 83-88 (1999)

The 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 the 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.

1998
 

1997
 

J.P.M. Camacho, J. Cabrero, M.D. López-León, M.W. Shaw Evolution of a near-neutral B chromosome pp. 301-318. In N. Henriques-Gil, J.S. Parker, M.J. Puertas (eds.), Chromosomes Today 12. Chapman & Hall, London (1997). UK.

 
 

A. J. Castro, M. C. Pardo, M. D. López-León, J. Cabrero, J. P. M. Camacho Mating frequency increases somatic condition but not productivity in Locusta migratoria females Hereditas 126: 53-57 (1997)

Three types of controlled crosses differing in mating frequency and male availability were performed with Locusta migratoria specimens caught in natural populations. The results revealed the absence of a significant relationship between mating frequency and female fitness measured as egg and embryo productivity during early reproduction, but weight-based somatic condition was significantly higher in females with higher mating frequency.

1996

M.D. López-León, J. Cabrero, J.P.M. Camacho Achiasmate segregation of X and B univalents in males of the grasshopper Eyprepocnemis plorans is independent of previous association. Chromosome Research 4: 43-48 (1996)

A B chromosome proved to be more frequent in males than females of the grasshopper Eyprepocnemis plorans collected from Jete population in 1992. The meiotic behaviour of the X and B univalents was analysed in a high number of 1B males collected at this population in 1991 and 1992, beside males from another population (Salobreña) for comparison. These two chromosomes showed a significant tendency to migrate to opposite poles in the Jete population, during the two 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 these two univalents during 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.

 
 

J. Cabrero, F. Pascual, J.P.M. Camacho Accidental twins in a monembryonic insect. Genome 39: 222-224 (1996)

A grasshopper egg was found containing two embryos which were both males and contained one B chromosome. Their development was delayed in comparison with that of the untwinned embryos from the same egg–pod, but their morphology was apparently normal. The possible mechanisms causing this unusual phenomenon are discussed.

 
 

J.A. Herrera, M.D. López-León, J. Cabrero, M.W. Shaw, J.P.M. Camacho Evidence for B chromosome drive suppression in the grasshopper Eyprepocnemis plorans Heredity 76: 633-639 (1996)

The grasshopper Eyprepocnemis plorans is polymorphic for both  a B–chromosome and a heterochromatic segment of chromatin on the smallest autosome. Females transmit these to their offspring more frequently after copulating with a male from a population without B's than after copulating with a male from their own population. Paternity analyses using the heterochromatic segment as a marker showed that the effect of male on transmission does not depend on fertilisation because it occurs even when all the eggs are fertilised by sperm from another mating. The possible mechanisms include behavioural differences in mating and transfer of substances affecting female meiosis in male ejaculate. The data support the idea that the B-chromosome is initially subject to meiotic drive in populations in which it has not previously existed, and that genes which suppress this drive are then selected.

 
 

M.D. López-León, J. Cabrero, J.P.M. Camacho Negatively-assorted gamete fertilization for supernumerary heterochromatin in two grasshopper species Heredity 76: 651-657 (1996)

The analysis of controlled crosses and gravid females in two grasshopper species has shown non-random gamete fertilization caused, in both cases, by a preference of ova and sperm to fuse with gametes carrying an allele different from theirs. Such a fertilization bias was observed for a B chromosome in Eyprepocnemis plorans and a supernumerary chromosome segment on the M7 autosome in Chorthippus jacobsi. In addition, a male-biased sex ratio was observed in E.plorans among the embryo offspring produced by 1B gravid females which had mated with a 1B male in the field, but no sex ratio distortion was apparent in the same type of crosses performed in the laboratory. The possible causes of these distortions, and the role negatively-assorted fertilization may play on the maintenance of these polymorphisms in natural populations are discussed.

 

F. Perfectti and L. Pascual Segregation distortion of isozyme loci in cherimoya (Annona cherimola Mill). Theoretical and Applied Genetics. 93: 440-446 (1996)

We have studied the allelic segregation of 13 isozyme loci in hand-fertilizated heterozygous cherimoya trees (Annona cherimola Mill.). We analyzed 63 combinations of locus x progeny and found non-Mendelian segregations in 12 cases. The sequential Bonferroni method revealed only 8 cases of non-Mendelian segregation, which then we investigated these with several chi-square tests to discover what processes were involved. Gametic selection appears to be the main process, although zygotic selection seems also to be involved

 

V.E. Vicente, O. Moreira-Filho, J.P.M. Camacho Sex-ratio distortion associated with the presence of a B chromosome in Astyanax scabripinnis (Teleostei, Characidae). Cytogenetics and Cell Genetics 74: 70-75 (1996)

Cytogenetic studies were conducted on 154 specimens of Astianax scabripinnis collected at three localities in the Campos do Jordao 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 the chromosome no. 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 and. 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.