Nucleocytoplasmic interactions stabilize ploidy level in wheat interspecific hybrids

The cytoplasm of Aegilops squarrosa L. (2n = 14; DD) is compatible with the nuclei of Triticum aestivum L. (2n = 42; AABBDD) and the pentaploid F1 (2x = 35; AABBD), but not with the euploid nucleus of Triticum turgidum L. (2n = 28; AABB). To identify D-genome chromosomes with the genes conditioning differential nucleocytoplasmic compatibility, ae and sq 5x F1, having cytoplasms of T. aestivum and Ae. squarrosa, respectively, were reciprocally crossed to the seven doubled ditelosomics (d-dts) of the D-genome chromosomes (including four control crosses of d-dts 1A, 1B, and sq 5x F1). The 32 progeny were examined for the presence or absence of unpaired telosomes, monosomes, and the maximum number of bivalents at the meiotic metaphase I in the pollen mother cells for comparison with the transmission of ae and sq male and ae and sq female gametes carrying different numbers of D-genome chromosomes. The sq gametes with 1D, 5D, and other D-genome chromosomes had a strong functional advantage. In contrast, the ae gametes with 1D and other D genome chromosomes, except 5D, had a functional disadvantage. The sq and ae 5x F1 transmitted chromosome 5D through 80.0 and 72.2% of the male and 58.6 and 44.8% of the female gametes, respectively. We concluded that the sq gametes tended to increase and ae gametes tended to reduce the ploidy levels of the progeny.Key words: preferential gamete transmission, alloplasmic wheat, Triticum, Aegilops, polyploidy, aneuploidy.

The DDK inbred strain as a model for the study of interactions between parental genomes and egg cytoplasm in mouse preimplantation development

The DDK strain of mice has unusual genetic properties. When females of this strain are crossed to males of other strains, they generally exhibit a very low fertility, whereas reciprocal crosses are fully fertile as are the intrastrain crosses. The observed low fertility results from early embryonic lethality, the F1 embryos dying around the late morula–early blastocyst stage. Nuclear transplantation experiments between hybrid eggs of BALB/c and DDK strains has shown that failure of F1(DDK ♀× BALB/c♂) embryos to develop is not due to the combination per se of maternal (DDK) and paternal (BALB/c) genomes but rather to an incompatibility between paternal (BALB/c) genomic contribution and DDK cytoplasm. This incompatibility does not occur between a female BALB/c pronucleus and the DDK cytoplasm, suggesting the involvement of a differential imprinting of parental genomes. Introduction of cytoplasts isolated from DDK 1- to 8-cell embryos into BALB/c♀×BALB/c♂ or BALB/c♀×DDK♂ embryos of the corresponding developmental stage demonstrate that the cytoplasm of DDK embryos prevents the formation of normal blastocysts through a specific interaction with the paternal component of the BALB/c diploid nucleus. Genetic and molecular studies are underway to try and isolate the gene(s) responsible for the failure of (DDK♀×BALB/c♂)F1 embryos. These experiments should help in our understanding of nucleocytoplasmic interactions and the respective roles of parental genomes in early embryonic development.

Isolation of Ht genome chromosome additions from polyploid Elymus trachycaulus (StStHtHt) into common wheat (Triticum aestivum)

A combination of cytological and biochemical methods were used to isolate and identify six Triticum aestivum 'Chinese Spring' – Elymus trachycaulus (= Agropyron trachycaulum, 2n = 28, genomes StStHtHt) Ht genome disomic and ditelosomic chromosome addition lines. Protein and morphological markers indicated that Elymus chromosomes 1Ht, 1Htp, 5Ht, 6Ht, 7Ht, and 7Htp have been added to the wheat genome. Two alloplasmic addition lines, 1Ht and 1Htp, were determined to have favorable nucleocytoplasmic interactions by the presence of vegetative vigor and fertility restoration. The gene(s) for vigor and fertility restoration were located on the short arm of 1Ht. The Elymus chromosomes of each line were found to affect plant morphology and fertility, with the exception of disomic addition 6Ht, which appeared similar to 'Chinese Spring'. Phenotypic differences between each line may be attributed to the expression of genes from specific Elymus chromosomes, the cumulative dosage of homoeoalleles, or nucleocytoplasmic interactions. These morphological traits, in combination with biochemical markers, provide evidence of the gene synteny relationships between the Elymus and Triticum species. Knowledge of the homoeologous relationships among wheat and Elymus chromosomes may be useful for the eventual transfer of disease-resistance genes from Elymus to wheat.Key words: Elymus, wheat, addition line, polyploidy.

Nucleocytoplasmic interactions in the mouse embryo

Fertilized mammalian ova consist of haploid genomes derived from both parents and cytoplasmic components inherited largely from the female parent. These three cellular compartments must successfully interact with each other and with their environment for development to proceed. These interactions require the transposition of nuclear and cytoplasmic products between cellular compartments with resultant alteration of gene transcription and the cytoplasmic expression of preformed or newly synthesized gene products. We have investigated nuclear/cytoplasmic interactions in the mouse embryo via the microsurgical transfer of nuclei and cytoplasm. Experiments have specifically examined the ability of nuclei from later developmental stages or from a different species to support development, volume relationships between nuclear and cytoplasmic compartments, and the nonequivalency of the maternal and paternal genomic contributions to development. The ability of egg cytoplasm to alter the function of a variety of embryonic and adult nuclei and the ability of these nuclei to support development has been extensively tested in nuclear transplantation investigations in amphibian embryos.