Cytogenetical studies onBougainvillea. I. a case of interchange heterozygosity

A case of interchange heterozygosity has been found inB. peruviana cv “Princess Margaret Rose” in which there is a regular formation of 15 bivalents and an interchange multiple of 4 chromosomes. The multiple is always associated with the nucleolus at diakinesis, indicating that one of the chromosome involved is nucleolar. The nucleolar pair of chromosomes shows a slight heteromorphicity which may be due to an unequal interchange. Although 80% of interchange multiples orientate non-disjunctionally, yet 65% pollen is stainable. The pollen is ineffective in self pollination, but highly effective in crosses with 2x and 3x cultivars ofB. spectabilis. The higher pollen stainability indicates that the deficiencies and duplications caused by non-disjunction do not have serious physiological offects on pollen grains and that its genome can withstand rearrangements.     

An interchange heterozygote inThelepogon elegans roth ex roem et schult.

In a population of 231 plants of the diploidThelepogon elegans (n=5), seven were found to be interchange heterozygotes. A multiple of four, six or eight chromosomes was observed in 62.32% of the pollen mother cells at MI, the remainder having five bivalents. Chiasmata were mostly localised terminally. The orientation of the multiples was predominantly alternate. The subsequent course of meiosis was normal, and pollen fertility was high.     

Genic heterozygosity, chromosomal interchanges and fitness in rye: any relationship?

Relationship between heterozygosity at allozyme loci, chromosomal interchanges and fitness was analyzed in a rye cultivar showing a polymorphism for such rearrangements. Nine allozyme systems (ACO, ACPH, GOT, GPI, LAP, MDH, PER, PGD and PGM) and five components of fitness (number of fertile tillers, total offspring, egg cell fertility, flowers/ear and seeds/ear) were studied. The estimated selection coefficients against interchange heterozygotes ranged from s = 0.12 to s = 0.34. A significant effect of the genic heterozygosity on some fitness components was observed in interchange heterozygotes (tillering and total offspring), in their standard homozygous sibs (flowers/ear and seeds/ear) and in the descendants of the crosses between standard karyotypes (flowers/ear, seeds/ear and egg cell fertility). However, the main effect was linked to genetic background associated to different crosses. Significant differences for Acph-1, Gpi-1, Lap-1, Mdh-1, Mdh-4, Pgd-2 and Pgm-1 loci were also found in some of these crosses although these differences were inconsistent. This suggests that probably the allozyme loci analyzed were not directly contributing to the fitness and that they are linked, in some cases, to different deleterious alleles depending on both cross and locus. This fact could support the local effect hypothesis as explanation although we do not discard the existence of some inbreeding level (general effect hypothesis) since all crosses and loci studied show a overall consistent trend of increased fitness with increased heterozygosity.     

A translocation heterozygote in garden Canna

A case of translocation heterozygosity has been discovered in gardenCanna. Morphologically the heterozygote is distinctive, being a pygmy type with small flowers which do not open. Cytologically it shows a ring or a chain of 4 plus 7 bivalents. The present observations reveal that rings disjunct usually in adjacent, and chains in an alternate manner. The subsequent course of meiosis is normal. The variety is totally sterile both after self and cross pollination. It has very likely arisen as a hybrid between two parents differing in one interchange.     

Fitness Characteristics and Allozyme Heterozygosity in an Artificial Population of the Sable Martes zibellina L.

Associations between some characteristics of fitness (the age of the first reproduction, life span, fertility, and the number of missed pregnancies) and heterozygosity for some allozyme loci have been studied in a population of the sable Martes zibellina L. kept in cages at a farm. Of all fitness characteristic studied, a weak correlation has only been found between heterozygosity and the age of the first reproduction (maturation rate). The age of the first reproduction is positively associated with the life span and fertility. The results obtained are discussed in terms of the relationship between the genetic variation of longevity and developmental rate and prospects of their application to breeding practice.     

Cytogenetics of pearl millet

Summary The somatic karyotype of pearl millet Pennisetum americanum (L.) Leeke. (2n = 14) has been studied in several cultivars, but few cytological markers have been discovered which could help in the easy identification of the chromosomes. Analysis of pachytene bivalents permits such identification but is feasible only in a few cultivars. Recently, several lines having telocentric chromosomes have been produced and classified but their potentialities as cytogenetic tools have yet to be explored. Some African populations of pearl millet carry B-chromosomes in their karyotype. Cytogenetics of B-chromosomes has been reported in great detail. Bs undergo spontaneous changes to produce deficient- and iso-chromosomes. The main effect of B-chromosomes is on chiasma frequency which is exerted by the relative amounts of chiasma promoting euchromatin and the chiasma depressing heterochromatin in the Bs. Haploid plants occur occasionally and sometimes show a low degree of seed set, offering a possibility of establishing homozygous inbred lines. Cytogenetics of several spontaneous and induced autotetraploids have been reported. In general quadrivalent formation between the seven sets of four homologues was random. Seed set of the autotetraploids could be improved by selection; improved seed fertility was found to be associated with increased chiasma frequency, increased quadrivalent frequency and regular distribution of chromosomes at anaphase I. Genes controlling morphological characters of plant phenotype segregate independent of those controlling fertility and in pearl millet polyploidy per se is not limiting to plant vigour. Primary trisomics represent the best studied among the aneuploids of pearl millet. All the seven primary trisomics have been identified and described. Some were used in assigning genes to specific chromosomes but in general trisomies have poor vigour and fertility, and show low frequency of transmission. Apart from B-chromosomes, cytogenetics of interchanges has been the best studied aspect of pearl millet. The frequency of co-orientation of an interchange complex at metaphase I, which determines the fertility or sterility of the interchange heterozygote, is influenced by the genetic background and thus is theoretically amenable for selection leading to improved fertility of the heterozygote. Interchange tester-stocks have been assembled which can be used to identify the chromosomes involved in any newly obtained interchange. A complex interchange line involving all the chromosomes of the complement has also been produced, but the ring-of-fourteen produces total male and female sterility.Genotypic control of mitosis and meiosis has been reported, with reference to chromosome numerical mosaicism, multiploid sporocytes, desynapsis and chromosome fragmentation, and male sterility. Pearl millet being a largely outbreeding species, forced inbreeding was mainly found to result in loss of morphological vigour and reduction in mean chiasma frequency per PMC. Interspecific hybrids between pearl millet and several related species have been cytologically investigated and homology of the seven chromosomes of pearl millet with seven of the fourteen chromosomes of P. purpureum has been demonstrated. Cytogenetic evidence from haploids, autopolyploids and interspecific hybrids has indications to suggest that the haploid number of x = 7 is derived from x = 5, but the evidence is inconclusive and needs critical evaluation.     

Long‐term selection for litter size in swine results in shifts in allelic frequency in regions involved in reproductive processes

High‐density genotype data were analyzed in three lines of swine that express substantial variation in sow fertility to uncover regions of the genome potentially influenced during selection for litter size traits. The experimental lines examined include the Nebraska Index Line (NIL), which has been subjected to long‐term selection for litter size; a control line derived from the same population that founded NIL; and a commercial Duroc × Hampshire (D × H) population, in which no selection for litter size was practiced. Regions of the genome potentially affected by selection for litter size traits in NIL were determined by multiple lines of evidence, including altered allelic frequency compared to the other lines, loss of heterozygosity and relative extended haplotype homozygosity. Additionally, a genome‐wide association study for litter size traits was conducted in a population based on NIL and commercial maternal line genetics. Several genomic regions identified as putative signatures of selection overlapped with QTL for litter size traits. One of these regions, located on SSC2 (13–14 Mb), includes the candidate gene P2X3R, which plays a role in implantation and sustained release of hormones associated with reproductive processes. Sequencing identified synonymous SNPs in P2X3R that are fixed in NIL but polymorphic with nearly equal frequencies in the D × H line, indicating a potential role of P2X3R in sow fertility. These results suggest that data derived from these lines can help to uncover and understand a portion of the genetic variance associated with fertility traits in swine.     

Heterozygosity and Fitness: No Association in Scots Pine

The association of six quantitative traits related to fitness with heterozygosity at 12 allozyme loci has been examined in three populations of Scots pine, Pinus sylvestris. Because of several characteristics of this organism and of this extensive data set, it appeared that this study would show a positive association between heterozygosity and these traits if indeed heterozygotes had higher values for these quantitative traits. Using several different statistical techniques including analysis of variance, regression with the scaling recommended from the adaptive distance model, and multiple regression, no evidence of an association was found. For example, only between 7 and 8% of the regression tests were significant at the 5% level and half of these showed a positive association and half showed a negative association. Further, the multiple regression analysis explained on average only 5.8% of the variation observed in the six different traits and only 1.5% of this variation was explained by a positive association. Power analysis was carried out (for the first time on these type of data), both for the single locus heterozygous advantage and the association of individual multiple locus heterozygosity and the quantitative traits. For diameter and height, two traits often used in similar studies, the average power to detect a single locus heterozygous advantage of 0.10 was 0.737 and the average power to detect a mean heterozygote advantage of 0.05 per locus for multiple loci was 0.797. As a result of this study and an examination of the published results from other studies, it appears that what positive associations have been observed are probably not, in large part, due to the presence of intrinsic heterozygote advantage.     

Genetic variability in Muscari comosum L. (Liliaceae) IV Geographical distribution and adaptive role of the polymorphic variants of chromosome 2

Muscari comosum L. (Liliaceae) has a chromosomal polymorphism for a pericentric inversion and a supernumerary chromosome segment probably due to an unequal interchange or insertional translocation. Both arrangements are widely distributed throughout the species range and the mean genetic distance among populations is D=0.131±0.075. There are no correlations between genetic distance and geographic distance or latitude. Only appreciable decreases in the frequencies of the inversion are detected in populations with ecologically marginal characteristics. There is a permanent and extended association between chromosomal inversion and an enzymatic locus (ADH). An excess of individuals heterozygous for the inversion was found and female productivity of heterozygotes is higher than that of corresponding homozygotes. A low rate of inversion heterozygosity in populations with ecologically marginal characteristics could be explained by natural selection. With respect to the adaptive role of the segment, although no homozygotes are found and may be selected against, heterozygotes could have heterotic effects.     

A spontaneous translocation heterozygote involving centromere regions in Gomphocerus sibiricus (L.) (Orthoptera: Acrididae)

A spontaneous interchange with breaks in the centromeric regions of the L₂ and L₃ submetacentric chromosome pairs is described, using the C-banding technique, in a male of the mountain grasshopper Gomphocerus sibiricus (2n=17). In 100% of the examined cells from standard follicles of the mutant, the interchange was present as a ring of four and concordant separations during anaphase-I were always observed. The orientation of these rings is predominantly either adjacent I or II and a very much lower frequency was detected for the alternate type. Fertility is therefore expected to be low. — A single follicle was found where the centromeric region of the 3² chromosome had split, resulting in two telocentric chromosomes. At meiosis chains with five centromeric regions resulted. The nature of the original interchange has been interpreted on the basis of this new aberration. — A large variation in the number of nucleoli is regularly present in spermatocytes at the same stage and differences between the two karyotypes (mutant and standard) could not be demonstrated using Ag-staining. This technique has also been used to study the morphology, with respect to number and localization of the adjunct centrioles, of the spermatids in the mutant.     

Born blonde: a recessive loss‐of‐function mutation in the melanocortin 1 receptor is associated with cream coat coloration in Antarctic fur seals

Although the genetic basis of color variation has been extensively studied in humans and domestic animals, the genetic polymorphisms responsible for different color morphs remain to be elucidated in many wild vertebrate species. For example, hypopigmentation has been observed in numerous marine mammal species but the underlying mutations have not been identified. A particularly compelling candidate gene for explaining color polymorphism is the melanocortin 1 receptor (MC1R), which plays a key role in the regulation of pigment production. We therefore used Antarctic fur seals (Arctocephalus gazella) as a highly tractable marine mammal system with which to test for an association between nucleotide variation at the MC1R and melanin‐based coat color phenotypes. By sequencing 70 wild‐type individuals with dark‐colored coats and 26 hypopigmented individuals with cream‐colored coats, we identified a nonsynonymous mutation that results in the substitution of serine with phenylalanine at an evolutionarily highly conserved structural domain. All of the hypopigmented individuals were homozygous for the allele coding for phenylalanine, consistent with a recessive loss‐of‐function allele. In order to test for cryptic population structure, which can generate artefactual associations, and to evaluate whether homozygosity at the MC1R could be indicative of low genome‐wide heterozygosity, we also genotyped all of the individuals at 50 polymorphic microsatellite loci. We were unable to detect any population structure and also found that wild‐type and hypopigmented individuals did not differ significantly in their standardized multilocus heterozygosity. Such a lack of association implies that hypopigmented individuals are unlikely to suffer disproportionately from inbreeding depression, and hence, we have no reason to believe that they are at a selective disadvantage in the wider population.     

Metaphase I orientation of chain-forming interchange quadrivalents: a theoretical consideration

The orientation behavior of chain forming interchange quadrivalents at metaphase I was studied in three interchange heterozygotes of pearl millet [Pennisetum americanum (L.) Leeke] which involve chromosomes 1, 3, 6 and 7 in various combinations. Of these, two combinations predominantly produced rings and the third was a chain-forming type. The chain quadrivalents derived from the two ring-forming interchanges, as well as the chain quadrivalent generated by the third interchange, all showed one adjacent orientation at metaphase I (adjacent-1 or -2, depending upon the formation or failure of chiasmata and their positions in the different segments of the pachytene cross). Homologous centromere co-orientation leading to adjacent-1 and alternate-1 occurs following chiasma failure in the noncentric arms of the pachytene cross, and nonhomologous centromere co-orientation leading to adjacent-2 and alternate-2 occurs following chiasma failure in the centric arms of the pachytene cross. Thus, it has been proposed that, unlike in ring quadrivalents, a specific chain quadrivalent will have only homologous or nonhomologous centromere co-orientations at metaphase I.     

Interchange quadrivalents and chromosome order at meiotic metaphase I in Briza L. (Gramineae)

In interchange heterozygotes of Briza humilis and B. media the interchange quadrivalent is shown to be preferentially positioned in flattened, lateral spreads at metaphase I. The positioning of the interchange quadrivalents is different in the two species but in both the frequency of alternate or adjacent orientation is different for different positions on the metaphase plate. B chromosomes in B. humilis are found to alter the positioning of the quadrivalent and the B chromosomes themselves are also found to show a nonrandom distribution on the metaphase plate.     

Genomic conflict drives patterns of X‐linked population structure in Drosophila neotestacea

Intragenomic conflict has the potential to cause widespread changes in patterns of genetic diversity and genome evolution. In this study, we investigate the consequences of sex‐ratio (SR) drive on the population genetic patterns of the X‐chromosome in Drosophila neotestacea. An SR X‐chromosome prevents the maturation of Y‐bearing sperm during male spermatogenesis and thus is transmitted to ~100% of the offspring, nearly all of which are daughters. Selection on the rest of the genome to suppress SR can be strong, and the resulting conflict over the offspring sex ratio can result in the accumulation of multiple loci on the X‐chromosome that are necessary for the expression of drive. We surveyed variation at 12 random X‐linked microsatellites across 16 populations of D. neotestacea that range in SR frequency from 0% to 30%. First, every locus was differentiated between SR and wild‐type chromosomes, and this drives genetic structure at the X‐chromosome. Once the association with SR is accounted for, the patterns of differentiation among populations are similar to the autosomes. Second, within wild‐type chromosomes, the relative heterozygosity is reduced in populations with an increased prevalence of drive, and the heterozygosity of SR chromosomes is higher than expected based on its prevalence. The combination of the relatively high prevalence of SR drive and the structuring of polymorphism between the SR and wild‐type chromosomes suggests that genetic conflict because of SR drive has had significant consequences on the patterns of X‐linked polymorphism and thus also probably affects the tempo of X‐chromosome evolution in D. neotestacea.     

Position and orientation in the metaphase equator of an interchange quadrivalent of hybrid rye

The position and orientation of an interchange quadrivalent in flattened lateral views of metaphase I were studied in pollen mother cells of hybrid rye. Five quadrivalent types showed three positional distributions in the equator, these distributions having elements both similar to and very different from those found previously for an interchange quadrivalent of Allium triquetrum.     

Multilocus heterozygosity and inbreeding in the Siberian jay

Because of its common negative association with fitness, inbreeding is a major concern in conservation biology. Traditionally it has been measured as individual inbreeding coefficient calculated from the pedigree, but recently multilocus heterozygosity estimates have become commonly used as proxies. However, theoretical and simulation studies have cast doubt on the validity of these surrogates especially when they are based on only a few molecular markers. Yet, empirical studies reporting the correlation between multilocus heterozygosity and inbreeding coefficient are rare. We studied this relationship in a wild Siberian jay (Perisoreus infaustus) population subject to a long-term field study over 30 years. The correlations between inbreeding coefficient and the employed heterozygosity measures—standardized heterozygosity and internal relatedness—based on 21 microsatellite loci were weak. These results together with results from theoretical and simulation studies caution against use of multilocus heterozygosity estimates to study inbreeding in natural populations.     

Tertiary trisomics of pearl millet (Pennisetum americanum (L.) K. Schum): its cytomorphology,fertility and transmission

Summary Nineteen tertiary trisomics were isolated from some translocation heterozygotes and interchange trisomics of pearl millet. Cytological analysis of these trisomics indicates that chromosome association of trivalents, univalents and pentavalents were frequent in all the trisomics. But their ratio varied from one trisomic to the next. Other associations were relatively infrequent. The relative frequencies of 6 pentavalent configurations observed in different trisomics were studied and their probable association with mode of fertility and transmission rates have been discussed.     

Multiple chromosomal interchanges in pearl millet

Summary Intercrossing and irradiation were successfully used in pearl millet (Pennisetum typhoides) to develop multiple interchanges involving up to the total complement of all the chromosomes in one complex. In interchange heterozygotes showing 12 + 1 II and 14, 90.9 and 87.8 per cent of the cells, respectively, had chromosome configurations other than that of 12 and 14 chromosomes. In general, the frequency of such cells resulting from breakdown of the expected complex configuration increased with the increase in the number of translocated chromosomes in the complex. The higher the number of chromosomes involved in the interchange ring, the higher were the pollen and ovule sterility. The results indicated that meiotic instability, deficiency-duplication gametes, and unequal distribution of chromosomes account for increased sterility of multiple interchange heterozygotes. Even though interchanges in pearl millet predominantly show the alternate type of segregation, sterility seems to be the major barrier for the exploitation of the multiple interchange method for gamete selection and the establishment of homozygous lines in this plant species.     

Heterozygosity correlates with body size,nest site quality and productivity in a colonial waterbird,the whiskered tern (Chlidonias hybrida,Aves: Sternidae)

Positive effects of individual heterozygosity on naturally selected traits have been reported in wild populations of many animal taxa. The aim of this study was to test whether heterozygosity predicts the quality of acquired nest sites and productivity in a colonially breeding waterbird, the whiskered tern (Chlidonias hybrida). For this purpose, 40 adult terns from a small, recently established population in Central Poland were typed at eight microsatellite loci. We demonstrate that individual heterozygosity is positively related to hatching success. We hypothesize that this association could be mediated by direct effects of heterozygosity on the competitive abilities of individuals. We found that more heterozygous terns tended to breed in better protected central parts of the colony, suggesting that they had capabilities of outcompeting less heterozygous individuals and relegating them to the less attractive peripheries of the colony. It was also demonstrated that the link between heterozygosity and individual abilities to acquire more attractive nest site could be mediated by the larger size of heterozygous individuals. Although no correlations between heterozygosity and different components of condition were found, there was a positive association between female heterozygosity and both clutch size and egg size. We suggest that demonstrated heterozygosity‐fitness correlations could be primarily caused by inbreeding depression in the studied whiskered tern population.     

Signs of fitness and allozyme heterozygosity in an artificial population of sable Martez zibellinna L

Associations between some characteristics of fitness (the age of the first reproduction, life span, fertility, and number of missed pregnancies) and heterozygosity for some allozyme loci have been studied in a population of the sable Martez zibellinna L. kept in cages at a farm. Of all fitness characteristic studied, a weak correlation has only been found between heterozygosity and the age of the first reproduction (maturation rate). The age of the first reproduction is positively associated with the life span and fertility. The results obtained are discussed in terms of the relationship between the genetic variation of longevity and developmental rate and prospects of their application to breeding practice.