Paternal regulation of seed development in wheat hybrids

Summary Diallel crosses among Triticum boeoticum (4 lines from different geographical areas), T.urartu, Aegilops squarrosa and Ae. speltoides exhibited reciprocal differences in hybrid seed morphology, endosperm development, and embryo viability. T. urartu and Ae. squarrosa as females with T. boeotiaum and Ae. speltoides lead to shrivelled inviable seed. T.boeoticum accessions as female with Ae.speltoides also lead to shrivelled seeds. The reciprocal crosses produced plump seeds which either resembled the maternal parent or showed size differences. By altering the endospermic genome ratios, hybrid seeds with 1 (PF)/1 (PM) showed extreme shrivelling whereas those with 4 (PF)/1 (PM) were medium shrivelled to plump. Genetic experiments involving hybrids of T. boeoticum, T. urartu and T. monococcum showed that a factor is present in pollen or male gametes, which shows dosage effect and which, by interacting with the maternal genome, leads to endosperm abortion.     

REPRODUCTIVE ISOLATION OF TRITICUM BOEOTICUM AND TRITICUM URARTU AND THE ORIGIN OF THE TETRAPLOID WHEATS

The diploid wheats Triticum boeoticum and T. urartu are sympatric with one another throughout the geographic range of the wild tetraploids. Reciprocal crosses between ecogeographic types within each diploid species gave viable seed, but interspecific crosses consistently gave viable seed only when T. boeoticum was the female parent. Apparently urartu cytoplasm in combination with the boeoticum genome resulted in nonviable seed. The endosperm failed to develop normally despite regular endosperm fertilization. The F₁ plants obtained were completely self sterile although they showed regular intergenomic pairing (7II) at meiosis. Presumably the accumulation of cryptic differences between the two closely related genomes under reproductive isolation accounts for this sterility. The same accumulated cryptic differences could largely account for the preferential diploid pairing in the tetrapolid wheats which presumably were derived from such hybrids by chromosome doubling. The behavior of reciprocal crosses between the diploids and tetraploids suggested that T. boeoticum contributed the cytoplasm to both of the wild tetraploid species.     

Genetic control of seed proteins in wheat

Summary Electrophoretic profiles of crude protein extracts from seed of F₁ hybrids and reciprocal crosses among diploid, tetraploid and hexaploid wheats were compared with those of their respective parental species. The electrophoretic patterns within each of three pairs of reciprocal crosses, T.boeoticum X T.urartu, T.monococcun X T. urartu and T.dicoccum X T. araraticum, were different from one another but were identical with those of their respective maternal parents. Protein bands characteristic of the paternal parents were missing in F₁ hybrid seed suggesting that the major seed proteins in wheat were presumably regulated by genotype of the maternal parent rather than by the seed genotype. However, in another three pairs of reciprocal crosses, T.boeoticum X T. durum, T.dicoccum X T.aestivum and T. zhukovskyi x T. aestivum, protein bands attributable to the paternal parents were present in the F₁ hybrid seeds indicating that the seed proteins were not always exclusively regulated by the maternal genotype. The expression of paternal genomes is presumably determined by dosage and genetic affinity of the maternal and paternal genomes in the hybrid endosperm. The maternal regulation of seed protein content is probably accomplished through the maternal control over seed size. The seed protein quality may, however, depend upon the extent of expression of the paternal genome.     

TRITICUM URARTU AND GENOME EVOLUTION IN THE TETRAPLOID WHEATS

The A genome of the tetraploid wheats (AABB, 2n = 28) shows 5-6 bivalents in crosses with Triticum boeoticum (2n = 14) and various Aegilops diploids (2n = 14). The B genome has never been similarly identified with any species, and is commonly thought to have been modified at the tetraploid level. Triticum boeoticum was presumably accepted as the A-genome donor because of its morphological similarity to the wild tetraploids and because it was formerly the only known wild diploid wheat. The B donor has been thought to be Ae. speltoides or another species of the Sitopsis section of Aegilops, but these diploids show pairing affinity with A rather than B. More recently, another diploid wheat, T. urartu, was found to be sympatric with T. boeoticum throughout the natural range of the tetraploids. The synthetic boeoticum-urartu amphiploid was virtually identical morphologically with the wild tetraploid wheats, whereas various boeoticum-Sitopsis amphiploids were markedly different. But the urartu genome, like those of T. boeoticum and Sitopsis, paired with A and not with B. However, cytological evidence also shows (1) that the genomes of any plausible parental combination pair with one another, (2) that the A and B genomes of the tetraploid wheats pair with one another in the absence of the gene Ph, and (3) that homoeologous chromosomes of the tetraploids have differentiated further, presumably as a result of diploidization. Consequently, chromosome pairing at Meiosis I can be expected to give ambiguous evidence regarding the identity of the tetraploid genomes with their parental prototypes. A hypothesis regarding the expected pairing affinities between tetraploid homoeologues that have differentiated from closely related parental chromosomes is advanced to explain the anomalous pairing behavior of the A and B genomes. Triticum boeoticum and T. urartu are inferred to be the parents of the tetraploid wheats.     

THE RELATIONSHIPS BETWEEN MALE AND FEMALE FERTILITY AND AMONG TAXA IN DIPLOID WHEATS

Pollen stainability appears to be a reliable indication of the ultimate seed set in diploid interspecific hybrid and backcross populations in Triticum L. The correlation between percent pollen stained and number of seeds set is positive and highly significant (r = 0.92). Estimates of male and female fertility in the hybrids and backcrosses are interpreted to indicate that the domesticated diploid Triticum monococcum L. and wild diploid T. boeoticum Boiss. em. Schiem, are one and the same species, and that T. urartu Tum. is not a variety of monococcum or boeoticum, but rather a separate species. The F₁ hybrids and backcrosses between monococcum and boeoticum are normally male and female fertile. The F₁ hybrids between monococcum and urartu are completely sterile and complete to partial sterility exists in backcrosses.     

Molecular characterisation of two novel starch granule proteins 1 in wild and cultivated diploid A genome wheat species

Starch synthase IIa, also known as starch granule protein 1 (SGP-1), plays a key role in amylopectin biosynthesis. The absence of SGP-1 in cereal grains is correlated to dramatic changes in the grains’ starch content, structure, and composition. An extensive investigation of starch granule proteins in this study revealed a polymorphism in the electrophoretic mobility of SGP-1 between two species of wheat, Triticum urartu and T. monococcum; this protein was, however, conserved among all other Triticum species that share the A genome inherited from their progenitor T. urartu. Two different electrophoretic profiles were identified: SGP-A1 proteins of T. urartu accessions had a SDS–PAGE mobility similar to those of tetraploid and hexaploid wheat species; conversely, SGP-A1 proteins of T. monococcum ssp. monococcum and ssp. boeoticum accessions showed a different electrophoretic mobility. The entire coding region of the two genes was isolated and sequenced in an attempt to explain the polymorphism identified. Several single nucleotide polymorphisms (SNPs) responsible for amino acid changes were identified, but no indel polymorphism was observed to explain the difference in electrophoretic mobility. Amylose content did not differ significantly among T. urartu, T. monococcum ssp. boeoticum and T. monococcum ssp. monococcum, except in one accession of the ssp. boeoticum. Conversely, several interspecific differences were observed in viscosity properties (investigated as viscosity profiles using a rapid visco analyzer—RVA profiles) of these cereal grains. T. monococcum ssp. boeoticum accessions had the lowest RVA profiles, T. urartu accessions had an intermediate RVA profile, whereas T. monococcum ssp. monococcum showed the highest RVA profile. These differences could be associated with the numerous amino acid and structural changes evident among the SGP-1 proteins.     

Morphological and molecular characterisation confirm that Triticum monococcum s.s. is resistant to wheat leaf rust

The three diploid wheat species Triticum monococcum, Triticum boeoticum and Triticum urartu differ in their reaction to wheat leaf rust, Puccinia triticina. In general, T. monococcum is resistant while T. boeoticum and T. urartu are susceptible. However, upon screening a large collection of diploid wheat accessions, 1% resistant T. boeoticum accessions and 16% susceptible T. monococcum accessions were found. In the present study these atypical accessions were compared with 49 typical T. monococcum, T. boeoticum and T. urartu accessions to gain insight into the host-status of the diploid wheat species for wheat leaf rust. Cluster analysis of morphological data and AFLP fingerprints of the typical accessions clearly discriminated the three diploid species. T. monococcum and T. boeoticum had rather-similar AFLP fingerprints while T. urartu had a very different fingerprint. The clustering of most atypical accessions was not consistent with the species they were assigned to, but intermediate between T. boeoticum and T. monococcum. Only four susceptible T. monococcum accessions were morphologically and moleculary similar to the typical T. monococcum accessions. Results confirmed that T. boeoticum and T. monococcum are closely related but indicate a clear difference in host-status for the wheat leaf rust fungus in these two species. Received: 7 November 2000 / Accepted: 31 March 2001     

Ploidy barrier to endosperm development in maize

Maize kernels inheriting the indeterminate gametophyte mutant (ig) on the female side had endosperms that ranged in ploidy level from diploid (2x) to nonaploid (9x). In crosses with diploid males, only kernels of the triploid endosperm class developed normally. Kernels of the tetraploid endosperm class were half-sized but with well-developed embryos that regularly germinated. Kernels of endosperm composition other than triploid or tetraploid were abortive.-Endosperm ploidy level resulting from mating ig/ig x tetraploid Ig similarly was variable. Most endosperms started to degenerate soon after pollination and remained in an arrested state. Hexaploid endosperm was exceptional; it developed normally during the sequence of stages studied and accounted for plump kernels on mature ears. Since such kernels have diploid maternal tissues (pericarp) but triploid embryos, the present finding favors the view that endosperm failure or success in such circumstances is governed by conditions within the endosperm itself.-Whereas tetraploid endosperm consisting of three maternal genomes and one paternal genome is slightly reduced in size but supports viable seed development, that endosperm having two maternal and two paternal chromosome sets was highly defective and conditioned abortion. Thus, development of maize endosperm evidently is affected by the parental source of its sets of chromosomes.     

Gliadin polymorphism in wild and cultivated einkorn wheats

To study the relationships between different species of the Einkorn group, 408 accessions of Triticum monococcum, T. boeoticum, T. boeoticum ssp. thauodar and T. urartu were analyzed electrophoretically for their protein composition at the Gli-1 and Gli-2 loci. In all the species the range of allelic variation at the loci examined is remarkable. The gliadin patterns of T. monococcum and T. boeoticum were very similar to one another but differed substantially from those of T. urartu. Several accessions of T. boeoticum and T. monococcum were shown to share the same alleles at the Gli-1 and Gli-2 loci, confirming the recent nomenclature that considers these wheats as different subspecies of the same species, T. monococcum. The gliadin composition of T. urartu resembled that of the A genome of polyploid wheats more than did T. boeoticum or T. monococcum, supporting the hypothesis that T. urartu, rather than T. boeoticum, is the donor of the A genome in cultivated wheats. Because of their high degree of polymorphism the gliadin markers may help in selecting breeding parents from diploid wheat germ plasm collections and can be used both to search for valuable genes linked to the gliadin-coding loci and to monitor the transfer of alien genes into cultivated polyploid wheats. Received: 8 July 1996 / Accepted: 12 July 1996     

High molecular weight glutenin subunit variation in wild and cultivated einkorn wheats (Triticum spp.,Poaceae)

Variation in high molecular weight (HMW) glutenin subunit composition among wild and cultivated einkorn wheats (2n = 2x = 14, AA) was investigated using one- (SDS-PAGE and urea/SDS-PAGE) and two-dimensional (IEF × SDS-PAGE) electrophoretic analyses. The material comprised 150 accessions ofTriticum urartu, 160 accessions ofT. boeoticum, 24 accessions ofT. boeoticum subsp.thaoudar and 74 accessions of primitive domesticatedT. monococcum from many different germplasm collections. The biochemical characteristics of HMW-glutenin subunits ofT. boeoticum andT. monococcum were highly similar to one another but distinctly different from those ofT. urartu. All the species analysed were characterised by large intraspecific variation and only three HMW-glutenin subunit patterns were identical betweenT. boeoticum andT. monococcum. Consistent with the distinct nature ofT. urartu, all its HMW-glutenin patterns were different from those found inT. boeoticum andT. monococcum. The differences detected between these species might reflect their reproductive isolation and are consistent with recent nomenclatural and biosystematic treatments that recogniseT. urartu as separate species fromT. boeoticum andT. monococcum. The presence of three distinct glutenin components in some accessions of the species studied seems to be evidence for the existence of at least three active genes controlling the synthesis of the HMW-glutenin subunits in the A genome of wild and primitive domesticated diploid wheats. Results indicate also that HMW-glutenin subunits could represent useful markers for the evaluation of genetic variability present in different wild diploid wheat collections and subsequently for their conservation and future utilisation.     

Genetic diversity in wild diploid wheats Triticum monococcum var. boeoticum and T. urartu (Poaceae)

Summary The genetic diversity of two wild diploid wheat species, Triticum monococcum var. boeoticum and T. urartu, was assessed using starch gel electrophoresis. Genetic diversity is uniformly low in both species. Number of alleles per locus was very low with a mean of 1.22 for T. monococcum var. boeoticum and 1.19 in T. urartu. Percentage of polymorphic loci was also low, with a mean of 19.71 for T. monococcum var. boeoticum and a mean of 18.35 for T. urartu. Mean gene diversity was low with a mean of 0.052 in populations of T. monococcum var. boeoticum and a mean of 0.040 in populations of T. urartu. Genetic affinities of the species and of populations were computed using Nei's identity index (NI). Overall genetic affinities of the two species are NI=0.697. The genetic affinities of different populations of a species are uniformly high with NIs ranging from 0.894 to 1.000 in T. monococcum var. boeoticum and from 0.898 to 1.000 in T. urartu.Research supported by the California Agricultural Experiment Station and the International Board of Plant Genetic Resources     

Some features of the inheritance of avenins,the alcohol soluble proteins of oat

Summary The inheritance of avenin components, the prolamins (or alcohol soluble proteins) of Avena, is studied by means of gel electrophoresis. Avenin is composed of rather similar proteins which appear as a polymorphic group from a biochemical point of view. After a first preliminary investigation it showed a surprisingly high interspecific variability. The average number of its constituents increases with the ploidy level but it still is much lower than that of wheat gliadin.The avenin electrophoretic patterns of 47 samples (F₄, F₅ or F₆ seeds) resulting from 3 hexaploid crosses are compared with the parental patterns. Four kinds of inheritance are observed. Roughly 50% of progeny profiles are identical to those of one of the parents. They are composed occasionally of partial sections of parental patterns. Complete additiveness occurs rather seldom. However, in one of the crosses a significant number of progeny samples show a band, one of the very slow moving constituents, which was not present in either of the parents.The study of avenin in F₁ seeds, arising from reciprocal crosses between two homozygous parent plants, shows a significant effect of maternal gene dose in the triploid endosperm.Because of both the variability and the relatively small number of avenin constituents, these results show that typical endosperm proteins such as oat prolamin constitute a useful tool for phylogenetic studies of the genus Avena.     

In vitro embryo rescue culture of F<Subscript>1</Subscript> progenies from crosses between diploid and tetraploid grape varieties

The purpose of this study was to understand factors affecting in vitro embryo rescue culture from hybrids between diploid and tetraploid varieties of grape in creation new triploid germplasm resources. The effects of different media, removal ages of immature seeds and reciprocal crosses of parents on the germination and seedling survival of immature seeds from crosses between diploid and tetraploid grape varieties by in vitro embryo rescue culture were investigated. The results indicated that the medium consisting of NN-1969 + IAA 1.75 mg l⁻¹ + GA₃ 0.35 mg l⁻¹ + CH 400 mg l⁻¹ + AC 2.0 g l⁻¹ was better than other media. The optimal removal age of immature seeds for the best development of embryos was 35–45 days after pollination (DAP). The percentage of germination (PG) for immature seeds and the percentage of seedling survival (PSS) for immature seeds for diploid varieties used as female parents were 10.72% and 4.35% higher than when tetraploid varieties were used as female parents respectively. A total of 41 hybrid progenies from eight combinations were obtained, made up of 17 diploid, 9 tetraploid, 14 aneuploid, and 1 triploid progeny as determined by root-tip chromosome identification. The triploid progeny was from Fujiminori (2n = 4x = 76) × Jingxiu (2n = 2x = 38). These results implied that it was feasible to extend the hybridization range of grape and to create new germplasm resources by in vitro embryo rescue based on the conventional hybridization. The NN-1969 medium supplemented with GA₃ and IAA was more propitious to the development of immature seeds sampled at about 45 DAP. It was easier to obtain plants using diploid as female parent, but triploid progeny was only obtained using tetraploid as female parent.     

Inter-embryo competition in the progeny of autotriploid Aloineae (Liliaceae)

In reciprocal crosses between diploid and triploid Aloineae the progeny are largely diploid or diploid plus one or two chromosomes, but in reciprocal crosses between triploids and tetraploids they are tetraploid or nearly so. Thus the triploids contribute circa haploid gametes to the progeny when crossed with diploids but circa diploid gametes when crossed with tetraploids. These results are compared with those of a number of earlier workers. It is concluded that the bias in the frequency of progeny types towards diploidy or tetraploidy, depending on the ploidy level of the plant which is crossed with the triploid, is caused by inter-embryo competition. Those embryos with an endosperm/embryo factor of 1.5, the value found in normal diploid/diploid crosses having triploid endosperms, are selected in preference to those with factors higher or lower than 1.5.Inter-gamete competition also occurs among the euploid and aneuploid gametes produced by the triploids. This is more pronounced on the male side, because the degree of survival of aneuploid pollen from the triploids into the next generation is much lower than that of aneuploid egg nuclei.Non-reduction in the triploids gives rise to occasional pentaploid progeny in crosses with tetraploids, but it is more probable that in diploid/triploid crosses tetraploid progeny are the products of non-reduction in the diploid.     

Evidence on the nature and origins of endosperm dosage requirements in Solanum and other angiosperm genera

Success of seed development following sexual crosses is primarily dependent on proper endosperm function and development. The failure to produce triploids, or triploid block in 4x×2x crosses served as the impetus for numerous studies of embryo and endosperm to attempt to explain cross failure. Early explanations were based upon a concept of a 232 ploidy balance between maternal tissue, endosperm, and embryo. Subsequent studies done with maize demonstrated that normal endosperm development in intraspecific maize crosses is dependent solely on having a 21 maternal to paternal genome dosage in the endosperm. These results have been modified and extended to solanaceous species in the form of an endosperm dosage system in which empirically determined factors must bear the same 21 relationship for crosses to succeed. Crossing behavior of these species suggest that the system is polygenically controlled and regulates both interspecific and intraspecific crosses. Endosperm dosage systems explain many aspects of species evolution, but the system appears to have originated as an ancient means of ensuring diploid fidelity.     

Microsatellite and flow cytometry analysis to help understand the origin of Dioscorea alata polyploids

Background and Aims Dioscorea alata

is a polyploid species with a ploidy level ranging from diploid (2n = 2x = 40) to tetraploid (2n = 4x = 80). Ploidy increase is correlated with better agronomic performance. The lack of knowledge about the origin of D. alata spontaneous polyploids (triploids and tetraploids) limits the efficiency of polyploid breeding. The objective of the present study was to use flow cytometry and microsatellite markers to understand the origin of D. alata polyploids.

Methods

Different progeny generated by intracytotype crosses (2x × 2x) and intercytotype crosses (2x × 4x and 3x × 2x) were analysed in order to understand endosperm incompatibility phenomena and gamete origins via the heterozygosity rate transmitted to progeny.

Results

This work shows that in a 2x × 2x cross, triploids with viable seeds are obtained only via a phenomenon of diploid female non-gametic reduction. The study of the transmission of heterozygosity made it possible to exclude polyspermy and polyembryony as the mechanisms at the origin of triploids. The fact that no seedlings were obtained by a 3x × 2x cross made it possible to confirm the sterility of triploid females. Flow cytometry analyses carried out on the endosperm of seeds resulting from 2x × 4x crosses revealed endosperm incompatibility phenomena.

Conclusions

The major conclusion is that the polyploids of D. alata would have appeared through the formation of unreduced gametes. The triploid pool would have been built and diversified through the formation of 2n gametes in diploid females as the result of the non-viability of seeds resulting from the formation of 2n sperm and of the non-viability of intercytotype crosses. The tetraploids would have appeared through bilateral sexual polyploidization via the union of two unreduced gametes due to the sterility of triploids.     

Insight into the genetic variability analysis and relationships among some Aegilops and Triticum species,as genome progenitors of bread wheat,using SCoT markers

We assessed the molecular genetic diversity and relationships among some Aegilops and Triticum species using 15 start codon-targeted (SCoT) polymorphism markers. A total of 166 bands amplified, of which 164 (98.79%) were polymorphic. Analysis of molecular variance and inter-population differentiation (Gst) indicated high genetic variation within the studied populations. Our analyses revealed high genetic diversity in T. boeoticum, Ae. cylindrica, T. durum and Ae. umbellulata, low diversity in Ae. crassa, Ae. caudata and Ae. speltoides, and a close relationship among Ae. tauschii, T. aestivum, T. durum, T. urartu, and T. boeoticum. Cluster analysis indicated 180 individuals divided into 8 genome homogeneous clades and 11 sub-groups. T. aestivum and T. durum accessions were grouped together, and accessions with the C and U genomes were grouped into the same clade. Our results support the hypothesis that T. urartu and Ae. tauschii are two diploid ancestors of T. aestivum, and also that Ae. caudata and Ae. umbellulata are putative donors of C and U genomes for other Aegilops species that possess these genomes. Our results also revealed that the SCoT technique is informative and can be used to assess genetic relationships among wheat germplasm.     

Diploid endosperm formation in Tulipa spp. and identification of a 1:1 maternal-to-paternal genome ratio in endosperms of T. gesneriana L.

Most Liliaceae plants have the tetrasporic Fritillaria-type embryo sac and normally form diploid embryos and pentaploid endosperms derived from a 4:1 maternal-to-paternal genome ratio (4m:1p) after double fertilization. Here we characterize embryo sac and endosperm formation in Tulipa spp. of Liliaceae. Chromosome analysis using seeds derived from 2x × 2x crosses of Tulipa gesneriana (2n = 2x = 24) identified diploid chromosome number in the endosperm. Similarly, flow cytometric analysis confirmed diploid endosperm formation in T. gesneriana, T. fosteriana (2n = 2x = 24) and T. greigii (2n = 2x = 24). To further study the possible mechanism of diploid endosperm formation, we made interploidy crosses of triploid (2n = 3x = 36) × diploid in which aneuploid seeds with various chromosome numbers (2n = 25–36) were produced. Again, flow cytometric analysis confirmed the same ploidy level in both embryos and endosperms at all aneuploidy levels, suggesting that only a single haploid polar nucleus contributes to endosperm formation at fertilization. Histological observation further confirmed the physical separation of two polar nuclei by a large vacuole in the Fritillaria-type embryo sac of T. gesneriana that appeared to prevent the fusion of the two polar nuclei that originated at the micropylar and chalazal ends before fertilization. Taken together, these results indicate that diploid endosperms (1m:1p) are normally formed in Tulipa spp. by fusion of the micropylar polar nucleus (n) and a spermatid (n) but not by normal triple fusion. We also show that tulip endosperm partially overcomes the triploid block mechanism that occurs in interploidy crosses. Based on these observations, the possible role of triple nuclear fusion in double fertilization is discussed.     

Embryo development in reciprocal crosses of Phaseolus vulgaris L. and P. coccineus Lam.

Summary Embryo development was examined in reciprocal crosses of Phaseolus vulgaris cv. Great Northern and P. coccineus cv. Scarlet Runner. The formation of abnormal (shrunken and underdeveloped) embryos constituted the primary crossing barrier between the two species when P. coccineus was the female parent. Plants of P. coccineus X P. vulgaris were obtained by embryo culture. Although the P. vulgaris X P. coccineus cross resulted in normal seed development, the fertility of the resulting hybrids was much lower (27%) than that of the reciprocal hybrids (81%). Three classes of F₂ embryos, normal, shrunken, and underdeveloped were formed on reciprocal F₁s and the frequencies did not differ between reciprocal populations. Thus, the interactions between embryo and endosperm and/or maternal parent rather than cytoplasmic-nuclear effects seem to be important in the determination of the extent of embryo growth. The examination of pollen fertility of F₂ plants and the development of F₂ and F₃ embryos suggests that the formation of abnormal embryos and reduced male fertility are independent events. The P. vulgaris — P. coccineus crosses may be useful in studying the possible involvement of interspecific differences in hormonal metabolism in the development of hybrid embryos.