Transmission distortion of t-haplotypes is due to interactions between meiotic partners

The T/t-complex of the mouse includes a series of recessive lethal and semi-lethal mutations but, despite such lethalities, mutant t-haplotypes are found in high frequency in wild mouse populations. This polymorphism is apparently maintained because heterozygous males preferentially transmit the t-bearing chromosome to their offspring. Despite many attempts to define the basis of the transmission ratio distortion, it has been unclear whether this is because t-bearing sperm have better than average fertilizing ability or whether +-bearing sperm in heterozygous males are rendered defective. To examine this point, we constructed male (XY in equilibrium XY) chimaeras containing +/+ and +/tw73 genotypes, marked respectively by albino and pigmented coat colours, and two isozyme variants. Such males produce a mixture of three different sperm types: +-bearing sperm from the +/+ genotype, +-bearing sperm from the +/t genotype, and t-bearing sperm from the +/t genotype. Appropriate matings can distinguish between these three types, and our data, reported here, show that t-bearing sperm in chimaeric mice maintain their advantage over their 'meiotic partners' but do not have any advantage over sperm from the +/+ genotype.

On the role of fibronectin during the compaction stage of somitogenesis in the chick embryo

During the early stages of somitogenesis in the chick embryo the presomitic cells in the segmental plate undergo compaction. The aggregation of segmental plate cells is stimulated by fibronectin. The stimulation of segmental plate cells to aggregate and undergo compaction can be effected in isolated segmental plate cells, in isolated segmental plates, and in intact embryos removed from the yolk. The fact that the segmental plate cells react with greater vigor to cellular fibronectin than to plasma fibronectin suggests a specific molecular mechanism in the initiation of somitogenesis.

Fibronectin distribution during somitogenesis in the chick embryo

Somite formation in vertebrates is a multi-stage process. From a relatively homogeneous rod of mesenchyme, the segmental plate, somites are formed in a repeating sequence. Cell-cell adhesion has been proposed as a causal factor in somitogenesis. This led to an analysis of fibronectin in the segmental plate with respect to the initiation of somitogenesis. The pattern of fibronectin distribution can be correlated with the initiation of somitogenesis in the anterior portion of the segmental plate. Fibronectin distribution was determined using a high resolution antibody localization technique. Differences in fibronectin distribution were verified with computer-assisted image analysis. The evidence presented supports the hypothesis that an increase in cell-cell adhesion is a significant factor in the initiation of somitogenesis.