European genetic study on rheumatoid arthritis: is there a linkage of the interleukin-1 (IL-1), IL-10 or IL-4 genes to RA?

The genetic predisposition for rheumatoid arthritis (RA) is only partly explained by the HLA locus and most genetic factors involved in the susceptibility (and/or severity) of the disease await further identification. The first European genome scan in RA families provided suggestive evidence for linkage with a region (3.1/3q13) on chromosome 3, but many other potential RA susceptibility genes have yet to be analysed.

AIMS

To perform a linkage analysis with microsatellite markers located in the vicinity of the interleukin-1 (IL-1) gene superfamily, the IL-10 gene and the IL-4 gene cluster which might be considered putative candidate loci for RA.

METHODS

107 Caucasoid European RA sibpairs from 90 nuclear families were genotyped for markers flanking the genes for the IL-1 superfamily, IL-10 and the IL-4 gene cluster. Linkage analysis based on the identity by descent (IBD) in affected siblings was analysed with the program SIBPALNA. Affected sibpairs were stratified according to the identity by state (IBS) for three markers in the HLA region (DRB1 oligotyping, D6S276 and TNFa microsatellites) and to the presence/absence of erosive disease on X-ray examination.

RESULTS

Analysis of the whole family set showed an excess of allele sharing for markers of the IL-1 gene cluster (IBD 60%; P = 0.012) but not for IL-10 or IL-4. After stratification, the evidence of linkage to IL-1 was restricted to HLA concordant sibpairs (n = 32; IBD 70%; P = 0.006). Some evidence of linkage to IL-10 was also observed in HLA concordant sibpairs (IBD 66%; P = 0.03) and in sibpairs with erosive disease (n = 61; IBD 62%; P = 0.02).

CONCLUSIONS

We found suggestive evidence of linkage of RA to the IL-1 locus. The increased linkage to IL-1 and IL-10 in HLA-identical sibs suggests a possible interaction between these cytokines and the HLA loci. Moreover IL-10 could interact with HLA factors in predisposing to erosive disease. These results need to be tested in additional families for consistency and replication.

Aicardi-Goutières syndrome: monogenic recessive disease, genetically heterogeneous disease, or multifactorial disease?

Aicardi-Goutières syndrome (AGS) is a severe progressive familial encephalopathy, which is usually diagnosed shortly after birth. Using the principle of homozygosity mapping, genome-wide screening of five consanguineous families was performed to search for a homozygous region shared by all affected individuals. A total of 364 markers with an average spacing of 9.9 cM were genotyped, but no homozygous region common to all affected individuals could be found. Regions of homozygosity in affected sibs could only be identified within each family individually. This may reflect genetic heterogeneity, possibly related to clinical heterogeneity, since several syndromes are clinically difficult to distinguish from AGS. Involvement of a small number of genes and/or of an external factor, such as infection, may also explain the absence of a homozygous region common to all affected individuals.

New susceptibility locus for rheumatoid arthritis suggested by a genome-wide linkage study

Rheumatoid arthritis (RA), the most common autoimmune disease, is associated in families with other autoimmune diseases, including insulin-dependent diabetes mellitus (IDDM). Its genetic component has been suggested by familial aggregation (lambdas = 5), twin studies, and segregation analysis. HLA, which is the only susceptibility locus known, has been estimated to account for one-third of this component. The aim of this paper was to identify new RA loci. A genome scan was performed with 114 European Caucasian RA sib pairs from 97 nuclear families. Linkage was significant only for HLA (P < 2.5.10(-5)) and nominal for 19 markers in 14 other regions (P < 0.05). Four of the loci implicated in IDDM potentially overlap with these regions: the putative IDDM6, IDDM9, IDDM13, and DXS998 loci. The first two of these candidate regions, defined in the RA genome scan by the markers D18S68-D18S61-D18S469 (18q22-23) and D3S1267 (3q13), respectively, were studied in 194 additional RA sib pairs from 164 nuclear families. Support for linkage to chromosome 3 only was extended significantly (P = 0.002). The analysis of all 261 families provided a linkage evidence of P = 0. 001 and suggested an interaction between this putative RA locus and HLA. This locus could account for 16% of the genetic component of RA. Candidate genes include those coding for CD80 and CD86, molecules involved in antigen-specific T cell recognition. In conclusion, this first genome scan in RA Caucasian families revealed 14 candidate regions, one of which was supported further by the study of a second set of families.

Localization of the gene for thiamine-responsive megaloblastic anemia syndrome, on the long arm of chromosome 1, by homozygosity mapping

Thiamine-responsive megaloblastic anemia, also known as "TRMA" or "Rogers syndrome," is an early-onset autosomal recessive disorder defined by the occurrence of megaloblastic anemia, diabetes mellitus, and sensorineural deafness, responding in varying degrees to thiamine treatment. On the basis of a linkage analysis of affected families of Alaskan and of Italian origin, we found, using homozygosity mapping, that the TRMA-syndrome gene maps to a region on chromosome 1q23.2-23.3 (maximum LOD score of 3.7 for D1S1679). By use of additional consanguineous kindreds of Israeli-Arab origin, the putative disease-gene interval also has been confirmed and narrowed, suggesting genetic homogeneity. Linkage analysis generated the highest combined LOD-score value, 8.1 at a recombination fraction of 0, with marker D1S2799. Haplotype analysis and recombination events narrowed the TRMA locus to a 16-cM region between markers D1S194 and D1S2786. Several heterozygote parents had diabetes mellitus, deafness, or megaloblastic anemia, which raised the possibility that mutations at this locus predispose carriers in general to these manifestations. Characterization of the metabolic defect of TRMA may shed light on the role of thiamine deficiency in such common diseases.

Genetic exclusion of 14 candidate genes in lipoatropic diabetes using linkage analysis in 10 consanguineous families

Lipoatropic diabetes (LD) is a rare recessive autosomal disorder, mainly characterized by lipoatrophy with alterations in lipid metabolism and extreme insulin resistance. To identify molecular defects responsible for this disease, we tested the implication of 14 candidate genes coding for proteins involved either in insulin action, i.e. insulin receptor, insulin receptor substrate 1, insulin-like growth factor I receptor, diabetes-associated ras-like protein (Rad), and glycogen synthase, or in lipid metabolism, i.e. lipoprotein lipase; apolipoproteins CII, AII, and CIII; hepatic lipase; hormone-sensitive lipase; the beta 3-adrenergic receptor; leptin; and fatty acid-binding protein 2. To this end, haplotype and linkage analyses using genotyping with microsatellites in 10 consanguineous families provided us with powerful genetic tools. Our results show that in most families, lod scores at a null recombination fraction were less than -2. Haplotype analysis also argues against the involvement of these genes in LD. This implies that mutations in these genes are unlikely to make a major genetic contribution to LD.

A linkage study of the N-methyl-D-aspartate receptor subunit gene loci and schizophrenia in southern African Bantu-speaking families

Both direct and indirect evidence implicate excitatory amino acid neurotransmission in the aetiology of schizophrenia. The data are particularly suggestive for N-methyl-D-aspartate (NMDA) neurotransmission. Four of the six genes coding for subunits of the neural NMDA receptor have been mapped. We have studied segregation and allele sharing of markers in these four regions in a sample of southern African Bantu-speaking families multiply affected with DSM-III-R schizophrenia. This population was chosen because anthropological and linguistic data suggest that it has diverged from a small initial population within the past 1000 years, making shared genetic aetiology more likely. We find positive LOD score maxima of 0.876 at a marker D9S1838 on chromosome 9q34.3 near the NMDAR1 central subunit gene, 0.758 at marker D17S784 on chromosome 17q25 near the NMDAR2C potentiating subunit gene, and 0.453 at marker D12S77 near the NMDAR2B gene on chromosome 12p12 when analysing affected samples only. Only the region of NMDAR2A, on chromosome 16p13, can be excluded in this population. There is evidence of increased allele sharing on chromosomes 9p34.3 and 17q25 using APM. Multipoint allele-sharing analysis using GENEHUNTER does not reject possible effects on chromosome 9q34.3, but does not support any involvement of chromosome 17q25. We propose that the NMDA receptor may be involved in the genetic predisposition to schizophrenia in this population through covariation in several of the subunits, which is consistent with the genetic models of the inheritance of the disease.

Assignment of the mulibrey nanism gene to 17q by linkage and linkage-disequilibrium analysis

Mulibrey nanism (MUL) is an autosomal recessive disorder with unknown basic metabolic defect. It is characterized by growth failure of prenatal onset, characteristic dysmorphic features, constrictive pericardium, hepatomegaly as a consequence of constrictive pericardium, yellowish dots in the ocular fundi, and J-shaped sella turcica. Hypoplasia of various endocrine glands, causing hormone deficiencies, is common. Here we report the assignment of the MUL gene, by linkage analysis in Finnish families, to a 7-cM region flanked by D17S1799 and D17S948 on chromosome 17q. Multipoint linkage analysis gave a maximum LOD score of 5.01 at loci D17S1606-D17S1853 and at D17S1604. The estimate of the critical MUL region was further narrowed to within approximately 250 kb of marker D17S1853 by linkage disequilibrium analysis. Positional candidate genes that belong to the growth hormone and homeobox B gene clusters were excluded. These data confirm the autosomal recessive inheritance of MUL and allow highly focused attempts to clone the gene.

Microsatellites in the HLA region: on overview

Microsatellites are repeats of a DNA base motif (1-6 bp, mostly CA repeats) up to 100 times; they are distributed regularly all over the genome. Many of them are polymorphic and their high polymorphism is based upon a variable number of repeats. They are widely used for genetic mapping, linkage analysis, population genetics, evolutionary studies and in forensic medicine. Such markers have also been described in the HLA region since 1991, and a growing interest in their potential applications is being expressed. The aims of this review are: 1) to outline the presently available information from literature and molecular databases concerning 53 microsatellites in the HLA region (localization, type of repeat, number of alleles, heterozygosity, primers used for amplification); 2) to address the question of technical pitfalls when using such markers; 3) to discuss specific features such as their mutation rate (10 (-3) to 10 (-6), which is higher than that reported for HLA genes, and their linkage disequilibrium with HLA alleles; 4) to present an integrated map of microsatellites and genes of this region; and 5) to provide a synopsis of their different applications in HLA-related fields (disease studies, population genetics, recombination point studies, HLA region mapping, transplantation) along with perspectives for future use. Although some HLA region microsatellites have already been applied to the analysis of more than 10 diseases, it is now evident that their use in population genetics and the determination of genomic compatibility in bone marrow transplantation represent growing areas of application.

A novel mutation in the potassium channel gene KVLQT1 causes the Jervell and Lange-Nielsen cardioauditory syndrome

The Jervell and Lange-Nielsen (JLN) syndrome (MIM 220400) is an inherited autosomal recessive disease characterized by a congenital bilateral deafness associated with a QT prolongation on the electrocardiogram, syncopal attacks due to ventricular arrhythmias and a high risk of sudden death. JLN syndrome is a rare disease, which seems to affect less than one percent of all deaf children. Linkage to chromosome 11p15.5 markers was found by analysing four consanguinous families. Recombinants allowed us to map the JLN gene between D11S922 and D11S4146, to a 6-cM interval where KVLQT1, a potassium channel gene causing Romano-Ward (RW) syndrome, the dominant form of long QT syndrome, has been previously localized. An homozygous deletion-insertion event (1244, -7 +8) in the C-terminal domain of this gene was detected in three affected children of two families. We found that KVLQT1 is expressed in the stria vascularis of mouse inner ear by in situ hybridization. Taken together, our data indicate that KVLQT1 is responsible for both JLN and RW syndromes and has a key role not only in the ventricular repolarization but also in normal hearing, probably via the control of endolymph homeostasis.

Codon 102 of the cardiac troponin T gene is a putative hot spot for mutations in familial hypertrophic cardiomyopathy

BACKGROUND

Familial hypertrophic cardiomyopathy is a phenotypically and genetically heterogeneous disease. In some families, the disease is linked to the CMH2 locus on chromosome 1q3, in which the cardiac troponin T gene (TNNT2) has been identified as the disease gene. The mutations found in this gene appear to be associated with incomplete penetrance and poor prognosis. Because mutational hot spots offer unique possibilities for analysis of genotype-phenotype correlations, new missense mutations that could define such hot spots in TNNT2 were looked for in unrelated French families with familial hypertrophic cardiomyopathy.

METHODS AND RESULTS

Family members were genotyped with microsatellite markers to detect linkage to the four known disease loci. In family 715, analyses showed linkage to CMH2 only. To accurately position potential mutations on TNNT2, its partial genomic organization was established. Screening for mutations was performed by single-strand conformation polymorphism analysis and sequencing. A new missense mutation, Arg102Leu, was identified in affected members of family 715 because of a G-->T transversion located in the 10th exon of the gene. Penetrance of this new mutation is complete; echocardiographic data show a wide range of hypertrophy; and there was no sudden cardiac death in this family.

CONCLUSIONS

The codon 102 of the TNNT2 gene is a putative mutational hot spot in familial hypertrophic cardiomyopathy and is associated with phenotypic variability. Analysis of more pedigrees carrying mutations in this codon is necessary to better characterize the clinical and prognostic implications of TNNT2 mutations.

European Gene Mapping Project (EUROGEM): breakpoint panels for human chromosomes based on the CEPH reference families. Centre d'Etude du Polymorphisme Humain

Meiotic breakpoint panels for human chromosomes 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 15, 17, 18, 20 and X were constructed from genotypes from the CEPH reference families. Each recombinant chromosome included has a breakpoint well-supported with reference to defined quantitative criteria. The panels were constructed at both a low-resolution, useful for a first-pass localization, and high-resolution, for a more precise placement. The availability of such panels will reduce the number of genotyping experiments necessary to order new polymorphisms with respect to existing genetic markers. This paper shows only a representative sample of the breakpoints detected. The complete data are available on the World Wide Web (URL http:/(/)www.icnet.uk/axp/hgr/eurogem++ +/HTML/data.html) or by anonymous ftp (ftp.gene.ucl.ac.uk in/pub/eurogem/maps/breakpoints).

Fucosyltransferase genes are dispersed in the genome: FUT7 is located on 9q34.3 distal to D9S1830

Synthesis of A, B, H, Lewis and related histo-blood group antigens is catalyzed by different fucosyltransferases. Enzymatic acceptor specificity and tissue expression permit the definition of 2 types of alpha-2-fucosyltransferases and 5 types of alpha-3-fucosyltransferases encoded by specific genes registered as FUT1 to FUT7. We have previously assigned FUT4 to 11q21, the cluster FUT1-FUT2 to 19q13.3 and the cluster FUT6-FUT3-FUT5 to 19p13.3. The last gene cloned (FUT7) encodes an alpha-3-fucosyltransferase expressed in leukocytes which synthesizes the sialyl Lĕ antigen, a selectin ligand. We have localized this gene by PCR assay using somatic cell hybrids, which retain rearrangements of chromosome 9 characterized in respect with the genetic microsatellite map, and then by screening a cosmid library. We assign FUT7 to chromosome band 9q34.3 telomeric to D9S1830 and close to the genes ABC2 and C8G.

A mutation in HERG associated with notched T waves in long QT syndrome

Long QT syndrome (LQT) is a genetically heterogeneous inherited disorder that causes sudden death from cardiac arrhythmia. Four loci have been mapped to chromosomes 3, 4, 7 and 11 and three specific mutated genes for LQT syndrome have been identified. LQT2 results from mutations in the human ether-a-gogo-related gene, HERG, a cardiac potassium channel, whose protein product likely underlies Ikd the rapidly activating delayed rectifier current. By SSCP analysis and direct sequencing, we determined a new missense mutation in the HERG coding sequence, a G to A transition at position 1681 resulting in the substitution of threonine for a highly conserved alanine at codon 561. This mutation, Ala561Thr, in the coding sequence of the fifth membrane-spanning domain (S5) of the HERG protein seems to convey a risk of cardiac events in affected family members. In addition to a prolonged T wave of low amplitude on the surface ECG, a distinctive biphasic T-wave pattern was found in the left precordial leads of all affected subjects with the Ala561Thr mutation regardless of age, gender and beta blocking therapy.

A comprehensive genetic map of the human genome based on 5,264 microsatellites

The great increase in successful linkage studies in a number of higher eukaryotes during recent years has essentially resulted from major improvements in reference genetic linkage maps, which at present consist of short tandem repeat polymorphisms of simple sequences or microsatellites. We report here the last version of the Généthon human linkage map. This map consists of 5,264 short tandem (AC/TG)n repeat polymorphisms with a mean heterozygosity of 70%. The map spans a sex-averaged genetic distance of 3,699 cM and comprises 2,335 positions, of which 2,032 could be ordered with an odds ratio of at least 1,000:1 against alternative orders. The average interval size is 1.6 cM; 59% of the map is covered by intervals of 2 cM at most and 1% remains in intervals above 10 cM.

[Congenital muscular dystrophy with merosin deficiency: clinical, histopathological, immunocytochemical and genetic analysis]

A selective deficiency of a specific laminin isovariant, merosin made of M, B1 and B2 chains, was found in a series of 17 patients affected with congenital muscular dystrophy (CMD). The merosin deficiency was complete in 15 cases, and almost complete in two cases. An overexpression of the laminin A chain was seen in these biopsies, while B1 and B2 chains were normally expressed. Comparison of the clinical data with a series of 18 "merosin-non deficient" cases showed that the "merosin-deficient" cases were forming a more homogenous group than the "non-deficient" one. Hypotonia, contractures, motor development delay were generally more severe in the "merosin-deficient" series of cases. Moreover, white matter alterations were seen in most cases explored by MRI or scan imaging. A genetic linkage with a 6q2 locus, corresponding to the M chain gene localization, was found in a panel of informative families from French and Turkish origin with "merosin deficient" CMD. "Merosin non-deficient" families did not map on this locus. So, the "merosin-deficient" CMD can be considered as a peculiar entity within the group of Congenital Muscular Dystrophies.

An STS-based map of the human genome

A physical map has been constructed of the human genome containing 15,086 sequence-tagged sites (STSs), with an average spacing of 199 kilobases. The project involved assembly of a radiation hybrid map of the human genome containing 6193 loci and incorporated a genetic linkage map of the human genome containing 5264 loci. This information was combined with the results of STS-content screening of 10,850 loci against a yeast artificial chromosome library to produce an integrated map, anchored by the radiation hybrid and genetic maps. The map provides radiation hybrid coverage of 99 percent and physical coverage of 94 percent of the human genome. The map also represents an early step in an international project to generate a transcript map of the human genome, with more than 3235 expressed sequences localized. The STSs in the map provide a scaffold for initiating large-scale sequencing of the human genome.

Readjusting the localization of long QT syndrome gene on chromosome 11p15

Long QT syndrome (LQT) is an autosomal dominant cardiac disease characterized by ventricular arrhythmia. A first locus for LQT has been identified on chromosome 11p15.5 (LQT1), closely linked to HRAS. To refine the location of LQT1, microsatellites were genotyped in 8 French families and the following order was determined: tel-HRAS-DRD4-D11S922-D11S4046- IGF2-INS-TH-D11S1318-D11S1323-D11S1338-D11S90 9-D11S1346-cen. By haplotype analysis, 12 crossing-over events were identified in affected and unaffected subjects, delineating the LQT1 candidate region to 7 cM. This new delineated localization between D11S1318 and D11S1323 is in a more centromeric region than previously thought and is 5 cM proximal to HRAS.

A third locus for hereditary haemorrhagic telangiectasia maps to chromosome 12q

Hereditary haemorrhagic telangiectasia (HHT) or Rendu-Osler-Weber disease is an autosomal dominant vascular disorder which associates epistaxis, mucocutaneous and visceral telangiectases, and recurrent haemorrhage with chronic anaemia and visceral shuntings. Recently, the tumour growth factor (TGF)-beta binding protein endoglin localized to 9q33-34 was identified as responsible for HHT in several large kindreds with pulmonary arteriovenous malformations (PAVMs). Additional linkage studies demonstrated that HHT is a genetically heterogeneous disorder with families unlinked to this region of 9q. In the families in which HHT was not linked to chromosome 9, less PAVMs were present. Furthermore, in one of these families, HHT was found linked to 3p22, where the TGF-beta II receptor is located. In this linkage study, we have analysed DNA from two families, in which HHT was unlinked to chromosome 9q and 3p, and PAVMs were absent, with a series of genetic markers on the centromeric region of chromosome 12. Using two-point linkage analysis, a significant lod score of Zmax = 7.86 at theta = 0.05 was obtained with the D12S85 microsatellite marker.

Relative positions of two clusters of human alpha-L-fucosyltransferases in 19q (FUT1-FUT2) and 19p (FUT6-FUT3-FUT5) within the microsatellite genetic map of chromosome 19

Five on the seven cloned human fucosyltransferase genes have been mapped to two clusters, one on 19q and the other on 19p. Comparative DNA sequence analysis showed the Généthon microsatellite D19S596 lies 2.2 kb downstream of the coding region of FUT1, indicating that the cluster comprising the closely linked FUT1 and FUT2 genes is located 4 cM distal to D19S412 (lod score 13.7) and 9 cM proximal to D19S571 (lod score 11.7). Polymorphic markers of FUT3, FUT5, and FUT6 were used for linkage analysis with 14 Généthon microsatellites in Indonesian families. These three loci constitute a cluster on 19p, located between the Généthon microsatellites D19S216 and D19S567, which are known to be only 1 cM distant from each other. Two cross-overs, one between FUT6 and FUT3 and the other between FUT3 and FUT5, suggest the gene order 19pter-D19S216-FUT6-FUT3-FUT5-D19S567++ +-cen. Comparison of genetic and physical maps suggests that the FUT6-FUT3-FUT5 cluster is located on 19p13.3 and the FUT1-FUT2 cluster on 19q13.3. FUT6, FUT3 and FUT5 genes share more than 85% homology and encode three similar, but distinct alpha(1,3) fucosyltransferases. FUT1 and FUT2 share about 70% homology and encode two distinct alpha(1,2)fucosyltransferases. No sequence homology was found between the genes of the two clusters. The members of each of these two clusters have probably emerged by duplication and divergent evolution of two unrelated ancestor genes.

Localization of merosin-negative congenital muscular dystrophy to chromosome 6q2 by homozygosity mapping

Congenital muscular dystrophies (CMD) are autosomal recessive, heterogeneous disorders. The commonest forms are the Fukuyama CMD (FCMD), associated with mental retardation and structural brain anomalies, and classical (occidental) CMD, with pure muscle expression. FCMD has been localized to chromosome 9q31-q33. Following the discovery of merosin deficiency in some CMD cases, we have localized, by homozygosity mapping and linkage analysis (Zmax = 5.6; theta = 0.0 for marker AFM127xb2) in four merosin-negative families a CMD gene in a 16 cM region of chromosome 6q2 in the region of the laminin M chain gene. In three consanguineous, merosin-positive, CMD families there was no linkage to either chromosome 6q2 or 9q31-q33.

Genetic, phenotypic, and environmental correlations in black medic,Medicago lupulina L., grown in three different environments

We have investigated the relationship between phenotypic and genetic correlations among a large number of quantitative traits (36) in three different environments in order to determine their degree of disparity and whether phenotypic correlations could be substituted for their genetic counterparts whatever the environment. We also studied the influence of the environment on genetic and phenotypic correlations. Twenty accessions (full-sib families) ofMedicago luPulina were grown in three environments. In two of these two levels of environmental stress were generated by harvesting plants at flowering and by growing plants in competition with barley, respectively. A third environment, with no treatment, was used as a control with no stress. Average values of pod and shoot weight indicate that competition induces the highest level of stress. The genetic and phenotypic correlations among the 36 traits were compared. Significant phenotypic correlations were obtained easily, while there was no genetic variation for 1 or the 2 characters being correlated. The large positive correlation between the genetic and phenotypic correlation matrices indicated a good proportionality between genetic and phenotypic correlations matrices but not their similarity. In a given environment, when only those traits with a significant genetic variance were taken into account, there were still differences between genetic and phenotypic correlations, even when levels of significance for phenotypic correlations were lowered. Consequently, it is dangerous to substitute phenotypic correlations for genetic correlations. The number of traits that showed genetic variability increased with increasing environmental stress, consequently the number of significant genetic correlations also increased with increasing environmental stress. In contrast, the number of significant phenotypic correlations was not influnced by the environment. The structures of both phenotypic and genetic matrices, however, depended on the environment, and not in the same way for both matrices.

Maternal inheritance of chloroplast genome and paternal inheritance of mitochondrial genome in bananas (Musa acuminata)

Restriction fragment length polymorphisms (RFLPs) were used as markers to determine the transmission of cytoplasmic DNA in diploid banana crosses. Progenies from two controlled crosses were studied with heterologous cytoplasmic probes. This analysis provided evidence for a strong bias towards maternal transmission of chloroplast DNA and paternal transmission of mitochondrial DNA in Musa acuminata. These results suggest the existence of two separate mechanisms of organelle transmission and selection, but no model to explain this can be proposed at the present time. Knowledge of the organelle mode of inheritance constitutes an important point for phylogeny analyses in bananas and may offer a powerful tool to confirm hybrid origins.

A molecular marker-based linkage map of diploid bananas (Musa acuminata)

A partial molecular linkage map of the Musa acuminata diploid genome is presented. This map is based on 58 RFLP, four isozyme and 28 RAPD markers segregating in an F2 population of 92 individuals. A total of 90 loci was detected, 77 of which were placed on 15 linkage groups while 13 segregated independently. Segregation distortions were shown by 36% of all loci, mostly favoring the male parent. Chromosome structural rearrangements were believed to be one of the main causes of these distortions. The use of genetic linkage data to further the genetic and evolutionary knowledge of the genus Musa, as well as to help improve the design of breeding strategies, is discussed.