Advances in pasture plant breeding for animal productivity and health.

Plant breeding has had a substantial effect on the productivity and health of ruminant animals in New Zealand by improving the quantity, quality and reliability of grazed temperate pastures. Genetic changes have affected annual pasture productivity, seasonal growth, digestibility, protein/energy balance, level of rumen undegradable protein, leaf properties affecting intake, resistance to foliar diseases, and reductions in compounds that have an adverse impact on the health, welfare and reproductive fertility of ruminant animals. Most plant improvement programmes have achieved genetic gains in excess of 1% per year for a variety of target traits, and these gains are likely to continue given the high genetic variation available within forage plants. Significant heritable variation exists to improve forage quality, particularly for soluble carbohydrate and fibre fractions in grasses, and in the rate at which these change during the season. Deleterious animal health and welfare effects can be alleviated through the use of non-toxic endophytes in grasses, that do not produce lolitrem B and ergovaline. Use of improved cultivars, with the appropriate management, can add value to animal products.

Ryegrass Host Genetic Control of Concentrations of Endophyte-Derived Alkaloids.

Endophytic fungi in pasture grasses produce alkaloids which affect invertebrate and vertebrate herbivores. While the competence to produce an alkaloid is a property of the fungus, the host plant may moderate fungal activity. Host genetic influence on endophyte activity was studied in perennial ryegrass (Lolium perenne L.) infected with a common strain of Neotyphodium lolii (Latch, Christensen & Samuels) Glenn, Bacon & Hanlin. Progeny seedling families of a partial diallel cross and their 12 parent clones were compared in a glasshouse experiment. Peramine and ergovaline concentrations were determined by high pressure liquid chromatography (HPLC), and intensity of endophyte infection was determined by enzyme-linked immunosorbent assay (ELISA). Concentrations of peramine and ergovaline and the amount of endophyte mycelium in plants varied between families, consistently across two glasshouse cells and (for the HPLC data) two harvests. There was no indication of any maternal effects. Host genetic control was evident in significant general combining ability effects and smaller specific combining ability effects. Parent-progeny correlation coefficients were high, and narrow-sense heritability was estimated as 0.70, 0.72, and 0.58 respectively for ergovaline, peramine, and ELISA. Further analysis indicated little interaction between loci, and no directional dominance. The three traits were correlated, indicating that 41 and 65% of the genetically controlled variation in ergovaline and peramine concentrations, respectively, was a function of mycelial mass. However, there were departures from these relationships. Host plant selection may enable development of pastures with controlled low levels of toxic but ecologically beneficial endophyte metabolites.

Heart specification in the Mexican axolotl (Ambystoma mexicanum).

The concept of the morphogenetic field has been used extensively in developmental biology. However, little is known about the mechanisms that partition these broad areas of tissue into the smaller areas which actually form the corresponding structures, and the remaining tissue. In the Mexican axolotl, the heart field forms as the anterior lateral plate mesoderm migrates over the underlying pharyngeal endoderm between stages 14 and 28. We have previously shown that both the mid-ventral and lateral walls of the pharyngeal cavity have considerable inductive capacity at stage 14. If this inductive capability, and the competence of the mesoderm to respond, is retained between stages 14 and 28, a much broader area of mesoderm would be induced than actually participates in heart development. In this paper, we use explant cultures to establish that pharyngeal endoderm retains its inductive activity, and that both pre-cardiac mesoderm and lateral plate mesoderm caudal to the pharyngeal cavity remain competent to respond to the induction throughout this period. We also map the specified region of the antero-lateral mesoderm between stages 14 and 28 by placing carefully measured areas of mesoderm in culture without inductive endoderm. We found that the region capable of initiating a spontaneous beat approximately doubles in size during this period. Since the specified region is larger than the actual heart primordium, some mechanism must exist to partition "induced" mesoderm into heart-forming and non-heart-forming areas.(ABSTRACT TRUNCATED AT 250 WORDS)

Is chemotaxis a factor in the migration of precardiac mesoderm in the chick?

The chick heart is formed from bilateral patches of presumptive cardiac mesoderm cells which migrate over the endoderm and fuse in the midline. We have tested the possibility that this migration is controlled, at least in part, by a chemotactic substance exuded by the anterior end of the endoderm. We have used chick/quail combinations to follow naturally marked cells during the course of their migration. Chimaeric embryos were formed by fusing together parts of chick and quail embryos of stage 5-6. Each embryo possessed two pairs of precardiac regions, the quail pair lying immediately anterior to that of the chick. These chimaeras were then explanted in embryo culture. In the event of chemotaxis, cells from the posterior end of the quail precardiac mesoderm might be expected to invade the chick area. Samples of explants and chimaeras were examined at intervals from 2 to 24 h, but in no case were cells found to have changed their direction of migration as a result of the proximity of anterior endoderm. It is concluded that this work does not provide evidence for a chemotactic attraction by the anterior end of the endoderm.