Replication of ColE2 and ColE3 plasmids: Two ColE2 incompatibility functions

Summary We have identified and localized two incompatibility determinants (IncA and IncB) within a 1.3 kb segment of ColE2 sufficient for autonomous replication. The IncA determinant is localized in a region shorter than 250 bp and expresses incompatibility against both ColE2 and ColE3. The region which determines sensitivity to the IncA determinant seems to overlap with the region specifying the IncA determinant. The expression of the trans-acting factor(s) specifically required for replication of ColE2 interferes with expression of the IncA determinant against ColE2 but not against ColE3. The IncA determinant might be at least partly responsible for the copy number control of the plasmid. The IncB determinant is localized in a 50 bp region (origin) which is sufficient for initiation of replication in the presence of the trans-acting factor(s). The IncB determinant is specific for ColE2 and seems to be due to titration of the trans-acting essential replication factor(s) by binding.     

Linkage between an incompatibility locus and a peroxidase isozyme locus (Prx 7) in rye

Summary Under controlled growth chamber conditions of 30 °C, seed set after selfing is possible in normally self-incompatible rye plants. Within selfed progenies produced by this method, plants homozygous at the peroxidase isozyme locus Prx 7 were crossed to heterozygous individuals. Segregation at the Prx 7 locus in progenies of these crosses provides clear evidence of a close linkage between Prx 7 and one of the two incompatibility loci in rye. A recombination fraction in the range of 0–2% was calculated from the segregation data. In rye, Prx 7 is linked with a phosphoglucoisomerase locus (Pgi). The similarity between the observations in Secale cereale and those made in Lolium perenne is discussed.     

棉属栽培种与野生种杂交的不亲和性

本文研究了棉属栽培种与野生种杂交的不亲和性,试验材料涉及5个染色体组,包括2个栽培种(陆地棉和中棉)和5个野生种(戴维逊氏棉、瑟伯氏棉、三裂棉、阿拉伯棉和比克氏棉)。以陆地棉作母本,异己花粉管在花柱中生长缓慢,有花粉管胚珠低于10%,陆地棉×戴维逊氏棉杂种胚在子叶期坏死。以中棉作母本,不亲和性主要表现在受精后的胚胎发育过程中。     

Unilateral incompatibility as a major cause of skewed segregation in the cross between Lycopersicon esculentum and L. pennellii

Skewed segregations are frequent events in segregating populations derived from different interspecific crosses in tomato. To determine a basis for skewed segregations in the progeny of the cross between Lycopersicon esculentum and L. pennellii, monogenic segregations of 16 isozyme loci were analyzed in an F₂ and two backcross populations of this cross. In the F₂, 9 loci mapping to chromosomes 1, 2, 4, 9, 10 and 12 exhibited skewed segregations and in all cases there was an excess of L. pennellii homozygotes. The genotypic frequencies at all but one locus were at Hardy-Weinberg equilibria. In the backcross populations, all except two loci exhibited normal Mendelian segregations. No post-zygotic selection model could statistically or biologically explain the observed segregation patterns in the F₂ and backcross populations. A pre-zygotic selection model, assuming selective elimination of the male gametophytes during pollen function (i.e., from pollination to karyogamy), could adequately explain the observed segregations in all three populations. The direction of the skewed segregations in the F₂ population was consistent with that expected based on the effects of unilateral incompatibility reactions between the two species. In addition, the chromosomal locations of 5 of the 9 markers that exhibited skewed segregations coincided with the locations of several known compatibility-related genes in tomato. Multigenic unilateral incompatibility reactions between L. esculentum pollen and the stigma or style of L. pennellii (or its hybrid derivatives) are suggested to be the major cause of the skewed segregations in the F₂ progeny of this cross.     

Rare symmetric and asymmetric Nicotiana tabacum (+) N. megalosiphon somatic hybrids recovered by selection for nuclear-encoded resistance genes and in the absence of genome inactivation

Following protoplast fusion between Nicotiana tabacum (dhfr) and N. megalosiphon (nptII) somatic hybrids were selected on the basis of dual resistance to kanamycin and methotrexate. Despite strong selection for parental nuclear-encoded resistances, only nine N. tabacum (+) N. megalosiphon somatic hybrids were obtained. A preferential loss of the parental N. tabacum nuclear and organelle genome was apparent in some plants in spite of the lack of genomic inactivation by the irradiation or chemical treatment of the parental protoplasts. Only six of the nine hybrids recovered possessed both parental profiles of nuclear RFLPs and isoenzymes. The remaining three hybrids were highly asymmetric with two being identical to N. megalosiphon except for minor morphological differences and rearranged or recombined mitochondrial DNAs (mtDNA), while the other one was distinguishable only by the presence of a rearranged or recombined mtDNA, and was therefore possibly a cybrid. Overall, eight somatic hybrids possessed rearranged or recombined mtDNAs and chloroplast inheritance was non-random since eight possessed N. megalosiphon-type chloroplasts and only one had N. tabacum chloroplasts. In contrast, using the same selection approach, numerous morphologically similar symmetric somatic hybrids with nuclear RFLPs and isozymes of both the parental species were recovered from control fusions between N. tabacum and the more closely related N. sylvestris. In spite of the low frequency of recovery of symmetric N. tabacum (+) N. megalosiphon hybrids in this study, one of these hybrids displayed a significant degree of self-fertility allowing for back-crosses to transfer N. megalosiphon disease-resistance traits to N. tabacum. Plant Research Centre Contribution No. 1579     

The use of the isoenzymic marker gene Got-1 in the recognition of incompatibility S alleles in apple

The S incompatibility system of apple was confirmed through the application of the gene Got-1 for glutamate oxaloacetate transaminase as a marker for the S locus. The 11S alleles proposed by Kobel et al. (1939) were confirmed through anomalous segregations for Got-1 observed in 14 semi-compatible crosses and regular segregations observed in 2 fully compatible crosses. The S allele genotypes of Idared (S ₃ S ₇), Cox (S ₅ S ₉) and Fiesta (S ₃ S ₅) were determined and found to fall within the original series. By associating parental incompatibility genotypes with the segregation of Got-1 alleles, we were able to deduce the coupling of S and Got-1 alleles in 9 varieties.     

Somatic hybridization of sexually incompatible petunias: Petunia parodii,Petunia parviflora

Summary Somatic hybrid plants were regenerated following the fusion of leaf mesophyll protoplasts of P. parodii with those isolated from a nuclear-albino mutant of P. parviflora. Attempts at sexual hybridization of these two species repeatedly failed thus confirming their previously established cross-incompatibility. Selection of somatic hybrid plants was possible since protoplasts of P. parodii would not develop beyond the cell colony stage, whilst those of the somatic hybrid and albino P. parviflora produced calluses. Green somatic hybrid calluses were visible against a background of albino cells/calluses, and upon transfer to regeneration media gave rise to shoots. Shoots and the resultant flowering plants were confirmed as somatic hybrids based on their growth habit, floral pigmentation and morphology, leaf hair structure, chromosome number and Fraction 1 protein profiles. The relevance of such hybrid material for the development of new, and extensively modified cultivars, is discussed.     

Evolution of incompatibility systems in plants: Complementarity and the mating locus in flowering plants and fungi

Summary The restriction of sexual pairing by a specificity gene is considered to be an ancient development in the plant kingdom. The diversity and general parallelism of incompatibility systems seen amongst the phyla at the present time can be rationalized in terms of the association of various derived forms of the ancestral specificity unit with differing spectra of accessory factors controlling sexual physiology in the different phyla. Sexual morphogenesis has become divided into distinct phases under the control of complementary genes. These phases are initiated by a regulatory system of Co-ordinator genes which control the order in which groups of morphogenetic genes are expressed during development. The entire sexual cycle will be completed only if all the complementary groups are activated in the appropriate sequence. The present article discusses essential features of the evolution of the breeding locus in different phyla. These features are consistent in themselves with the present data and are not dependent on the proposed ancient origin of the specificity gene.The above hypothesis throws light on the (1) evolution of the complex mating loci in flowering plants and fungi; (2) evolution of complementary incompatibility and heteromorphic incompatibility in flowering plants; (3) anomalous cross-compatibility behaviour of mutants in the fungus Schizophyllum commune; (4) nature of homothallism in higher fungi; (5) mode of origin of new functional self-incompatibility alleles; and (6) homogenic and heterogenic incompatibility.     

Endosperm dosage relationships among Lycopersicon species

Summary Accessions of eight Lycopersicon species and five yellow-flowered Solanum species were used as males in crosses with 2x and 4x L. esculentum to observe seed set and progeny ploidy. Species which failed in crosses to L. esculentum were crossed as males to 2x and 4x L. peruvianum. In cases of low seed set, chromosome counts were undertaken to establish the nature of the progeny. Endosperm Balance Number (EBN) relationships were determined for the crossability groups. Results support the basic concept of an L. esculentum crossability complex and an L. peruvianum crossability complex. Within the L. esculentum complex, all EBNs appear identical with a value of 2. Within the L. peruvianum complex, more variability appears to exist. The EBN values of this group are higher, and may be approximately double those of the L. esculentum complex. The EBN of L. peruvianum var humifusum appears to be somewhat lower than other L. peruvianum types. The EBN values of S. lycopersicoides, S. rickii, S. ochranthum and S. juglandtfolium could not be determined experimentally. Differential aspects of Lycopersicon and tuber-bearing Solanum evolution may be interpreted on the basis of endosperm compatibility.Co-operative investigation of the Vegetable Crops Research Unit, U.S. Department of Agriculture, Agricultural Research Service, and the Wisconsin Agricultural Experiment Station     

Irregular segregation at the Pr locus controlling plastid inheritance in Pelargonium: gametophytic lethal or incompatibility system?

Summary Two distinct segregation patterns are recognized after G X W plastid crosses in Pelargonium. Type I parents produce offspring in which maternal zygotes are frequent, biparental intermediate, and paternal zygotes rare (MZ>BPZ>PZ), as defined by the presence or absence of green or white plastids in the young embryos into which the zygotes develop. Type II parents produce offspring in which maternal and paternal zygotes are frequent with biparental zygotes the least frequent class (MZ>BPZPr ₁ Pr ₁. Type II plants, which do not breed true, are regarded as heterozygotes — Pr ₁ Pr ₂. The nuclear gene is symbolized as Pr as it is presumed to control alternative patterns of plastid segregation through an effect on plastid replication.Selfs and intercrosses of heterozygous plants segregate in an unexpected 1:1 ratio and not the expected 3:1 (1:2:1). The alternative homozygote — Pr ₂ Pr ₂ — could not be detected. Reciprocal crosses between heterozygotes (Pr ₁ Pr ₂) and homozygotes (Pr ₁ Pr ₁) give the expected 1:1 ratio when the Pr ₂ allele is derived from the male, whereas there is often, but not always, a highly significant deviation from 1:1 when the Pr ₂ allele is derived from the female.A simple explanation, which is not wholly satisfactory, is to assume that Pr ₂ is a gametophytic lethal on the female side. An alternative, or additional, explanation is that an incompatibility mechanism is involved in which Pr ₁ is a self-compatible allele, Pr ₂ a self-incompatible allele, and Pr ₁-Pr ₂ cross-compatible alleles. Successful fertilization is then determined by sporophytic control on the male side and gametophytic control on the female side.