Relationship between heterosis and heterozygosity at marker loci: a theoretical computation

Summary In this paper we have studied the linear correlation between a genetic distance index between two parent lines (based on marker loci information) and the heterosis observed in the F₁ hybrid from the two lines, for a quantitative character (determined by several loci, or QTL). Theoretical computations of the correlation coefficient () between the distance index and the heterosis were made, assuming the biallelic model (defined by Fisher). When the alleles at both marker loci and QTL are equally distributed among the whole population of considered lines, the coefficient is a function of the squares of linkage disequilibria between alleles at marker loci and alleles at QTL. The QTL that are not marked by marker loci and marker loci that do not mark any QTL play symmetrical roles and can decrease greatly. We conclude that the prediction of F₁ hybrid heterosis based on marker loci would be more efficient if these markers were selected for their relationship to the alleles implicated in the heterotic traits considered.     

Theobroma cacao L.: a genetic linkage map and quantitative trait loci analysis

A genetic linkage map of Theobroma cacao (cocoa) has been constructed from 131 backcross trees derived from a cross between a single tree of the variety Catongo and an F1 tree from the cross of Catongo by Pound 12. The map comprises 138 markers: 104 RAPD loci, 32 RFLP loci and two morphologic loci. Ten linkage groups were found which cover 1068 centimorgans (cM). Only six (4%) molecular-marker loci show a significant deviation from the expected 11 segregation ratio.The average distance between two adjacent markers is 8.3 cM. The final genome-size estimates based on two-point linkage data ranged from 1078 to 1112 cM for the cocoa genome. This backcross progeny segregates for two apparently single gene loci controlling (1) anthocyanidin synthesis (Anth) in seeds, leaves and flowers and (2) self-compatibility (Autoc). The Anth locus was found to be 25 cM from Autoc and two molecular markers co-segregate with Anth. The genetic linkage map was used to localize QTLs for early flowering, trunk diameter, jorquette height and ovule number in the BC₁ generation using both single-point ANOVA and interval mapping. A minimum number of 2–4 QTLs (P<0.01) involved in the genetic expression of the traits studied was detected. Coincident map locations of a QTL for jorquette height and trunk diameter suggests the possibility of pleiotropic effects in cocoa for these traits. The combined estimated effects of the different mapped QTLs explained between 11.2% and 25.8% of the phenotypic variance observed in the BC₁ population.     

Relationships among genetic distance,forage yield and heterozygosity in isogenic diploid and tetraploid alfalfa populations

Isogenic diploid and tetraploid alfalfa (Medicago sativa L.) was studied with molecular markers to help understand why diploid performance and breeding behavior does not always predict that of tetraploids. In a previous study of partially heterozygous alfalfa genotypes, we detected a low correlation between yields of isogenic diploid (2x) and tetraploid (4x) single-cross progenies, and genetic distances were more highly correlated with yields of tetraploids than diploids. These differences may be related to the level of RFLP heterozygosity expected among progenies derived from heterozygous parents at the two ploidy levels. The objectives of this study were to determine the relationships among genetic distance, forage yield and heterozygosity in isogenic 2 x and 4 x alfalfa populations. Four diploid genotypes were chromosome doubled to produce corresponding isogenic autotetraploids, and these genotypes were mated in 4 × 4 diallels to produce 6 single-cross families at each ploidy level for field evaluation. Allele compositions of parents were determined at 33 RFLP loci by monitoring segregation of homologous restriction fragments among individuals within progenies, and these were used to estimate RFLP heterozygosity levels for all single-cross progenies at both ploidy levels. RFLP heterozygosity rankings were identical between progenies of isogenic diploid and tetraploid parents; but significant associations (P < 0.05) between estimated heterozygosity levels and forage yield were detected only at the tetraploid level. Since tetraploid families were nearly 25% more heterozygous than the corresponding diploid families, inconsistencies in the association between molecular marker diversity and forage yields of isogenic 2 x and 4 x single crosses may be due to recessive alleles that are expressed in diploids but masked in tetraploids. The gene action involved in heterosis may be the same at both ploidy levels; however, tetraploids benefit from greater complementary gene interactions than are possible for equivalent diploids. Present address: AgResearch Grasslands, New Zealand Pastoral Agriculture Research Institute, Palmerston North, New Zealand     

A test of the maximum heterozygosity hypothesis using molecular markers in tetraploid potatoes

It has been theorized that in cross-pollinated polyploid species hybrid vigor is maximized by the frequent occurrence of more than two alleles per chromosomal locus. In polyploid crops this condition of maximum heterozygosity has been reported to be associated with increased yield and optimum field performance. We report herein the first direct test of the maximum heterozygosity hypothesis. Molecular markers were used to examine the association between maximum heterozygosity and several components of yield in three different populations of tetraploid potatoes. The results indicate that the value of maximum heterozygosity is not universal but dependent on the genetic background of the material under evaluation. In a cross between adapted breeding lines, homozygosity was negatively correlated with tuber yield, and maximum heterozygosity was positively correlated with the proportion of tuber yield in the large-size fraction. In contrast, in crosses between adapted and unadapted parents, maximum heterozygosity had no detectable effect on any character. Quantitative trait locus (QTL) analysis of the three populations reveals that, regardless of the genetic background, additive genetic effects are more strongly correlated with the components of yield than are any measures of heterozygosity and that some common QTLs may be influencing yield in all three populations.     

Detection of a highly heterozygous locus in recombinant inbred lines of rice and its possible involvement in heterosis

Forty-seven recombinant inbred (RI) lines derived from a cross between two indica rices, cv Phalguna and the Assam land race ARC 6650, were subjected to restriction fragment length polymorphism (RFLP) analysis using cloned probes defining 150 single-copy loci uniformly dispersed on the 12 chromosomes of rice. Of the probes tested, 47 detected polymorphism between the parents. Heterozygosity was calculated for each line and for each of the polymorphic loci. Average heterozygosity per line was 9.6% but was excessive (>20%) in the 5 lines that seemed to have undergone outcrossing immediately prior to harvest. Average heterozygosity detected by each probe across the 47 RI lines was 9.7%. The majority of probes revealed the low level of heterozygosity (<8%) expected for F₅-F₆ lines in a species showing about 5% outbreeding. On the other hand, 7 probes exhibited heterozygosity in excess of 15%, while with a eighth probe (RG2 from chromosome 11) heterozygosity varied according to the restriction enzyme employed, ranging from 2% with SaII to 72% with EcoRV. The presence of 34 recombination sites in a segment of the genome as short as 24 kb indicates a strong selection for recombination between two neighbouring loci, one required as homozygous for the Phalguna allele, and the other heterozygous. Since selection was principally for yield advantage over that of the high-yielding parent, Phalguna, one or both of these loci may be important for heterosis in this cross. The results also indicate that heterozygosity as measured by RFLP can depend on the particular restriction endonuclease employed.     

A vigorous mutant sugarcane (Saccharum sp.) clone Co 527

Summary A vigorous fast growing mutant which ends vegetative growth sixty days earlier than the parent variety Co 527 was isolated from gamma irradiated vegetative buds. The mutant initially segregated but stabilized in the vM₄ generation. Its growth rate was almost fifty per cent higher than Co 527 beginning in the early stages of growth. It produced a significantly higher early shoot population which enabled it to yield a higher number of millable canes at maturity. Economic characters like sucrose content and juice purity remained unaffected. This mutant had a chromosomal basis in that the number was 2 to 3 lower than in the parent variety.Research carried out under the FAO/IAEA Co-ordinated Research Programme on the Improvement of Vegetatively Propagated Plants and Tree Crops through Induced mutations (Research Contract No. 1680)     

Biochemical diversity and genetic distance in two species of the genusSaguinus

A comparative study of 20 blood genetic systems was performed on three populations of genusSaguinus: S. fuscicollis weddelli, S. midas niger, andS. midas midas. Some markers are useful for the characterization of the two species.ADA ⁵, ADA⁶, CA2³, CA2⁴, andES2 ² occur only inS. fuscicollis whileADA ⁴is fixed inS. midas. S. midas midas showed heterozygosity value comparable to those previously obtained for the genusAlouatta. Estimates of genetic distance betweenS. fuscicollis andS. midas species of about 14% are in accordance with those referred to in the literature for interspecific differences. Genetic distances between subspecies ofSaguinus midas were nearly 3%, which is within the range for subspecies.     

Allozyme diversity in pheasants (<Emphasis Type="Italic">Phasianus</Emphasis> spp.) from breeding stations in Serbia

The pheasant breeds are widely used for restocking of natural populations depleted by hunting. The pheasant population number decline was detected during the 1970s in many hunting areas of Europe. One of its possible reasons might be the loss of adaptability in populations originating from breeding stations, which was caused by inbreeding depression. The aim of this paper was the analysis of genetic variability in pheasant populations from three breeding stations in Vojvodina province (Serbia) by means of allozyme diversity detection. The allozyme variability analysis of pheasants from all three breeding stations revealed polymorphisms at nine loci: Ldh-1, Mor-1, Mor-2, Es-1, Mod-2, Pgd, Gpi-2, Odh, and Sod. The analysis of individuals from three different breeding stations showed mean values of observed heterozygosity of H _o=0.137, polymorphism P _95%=30%, and H/P ratio H/P=0.430, which indicate a normal level of genetic variability for bird populations. Comparative analysis of three pheasant populations showed a high level of interpopulation differentiation.     

AFLP-based estimation of 2n gametophytic heterozygosity in two parthenogenetically derived dihaploids of Rosa hybrida L

Two dihaploid Rosa hybrida L. genotypes, derived through parthenogenesis by using irradiated pollen, were crossed with clonally propagated plants of the diploid species Rosa rugosa Thunb. and Rosa wichuraiana Crép., respectively. Three progeny groups were obtained which contained numerous polyploids, as determined by flow cytometry. Production of fertile 2n female gametes is apparently very common in one of these R. hybrida dihaploid derivatives, whereas the other one is able to produce fertile 2n pollen. Hence, an amplified fragment length polymorphism (AFLP) study was performed on the parental plants and the resulting hybrid offspring in order to estimate (1) the respective genomic parental contributions, and (2) the level of heterozygosity transmitted by the 2n unreduced gametes. Comparison of the levels of transmitted parental heterozygosity revealed that two types of 2n gametes were produced simultaneously, presumably resulting from restitution at the first and at the second meiotic division, respectively. Received: 15 February 2001 / Accepted: 22 May 2001     

Coalescent Size Versus Coalescent Time with Strong Selection

This work studies the coalescent (ancestral pedigree, genealogy) of the entire population. The coalescent structure (topology) is robust, but selection changes the rate of coalescence (the time between branching events). The change in the rate of coalescence is not uniform, rather the reduction in the time between branching events is greatest when the coalescent is small (immediately after the common ancestor is the only member of the coalescent) with little change when the coalescent is large (immediately preceding when that common ancestor becomes fixed and the size of the coalescent is N). This provides that the reduction in the coalescent time due to selection is much greater than the reduction in the cumulative size of the coalescent (total number of ancestors of the present population after and including the most recent common ancestor) due to selection. If Ns≫1, the coalescent and fixation times are approximately equal to , which is much less than the value N which would result from neutral drift (N rather than the canonical haploid neutral fixation time 2N is the appropriate comparison for the model considered here), in particular, it is 70% less for Ns=10 and 95% less for Ns=100. However, for those values of Ns, and N ranging between 10³ and 10⁶, the reduction in the cumulative size of the coalescent of the entire population compared to the neutral case ranges from 17% to 65% (depending on the values of N and s). The coalescent time for two individuals for Ns of 10 and 100 is reduced by approximately 70% and 94%, respectively, compared with the neutral case. Because heterozygosity is proportional to the coalescent time for two individuals and the number of segregating alleles is proportional to the cumulative size of the coalescent, selection reduces heterozygosity more than it reduces the number of segregating alleles.