R. Snowball

Strategies for developing a core collection of bladder clover (Trifolium spumosum L.) using ecological and agro-morphological data

Core collections are a way to improve the functioning of germplasm collections through systematic evaluation, exploitation, and improved management of the whole collection. The published sizes of cores have ranged from 10 to 30% of the collection. The first step in developing a core collection is usually stratification of the whole collection. We adopted three approaches in the process of core designation. First, the existing ecological data and newly acquired agro-morphological (evaluation) data were analysed to determine the optimal method for capturing the ecological and morphological diversity of accessions. Second, different selection strategies were compared to identify the most powerful analysis with the greatest likelihood of covering maximum diversity. Third, the correlation between the two datasets was investigated. The annual pasture legume, bladder clover (Trifolium spumosum L.), was used for this pilot study because 80% of the existing 398 accessions have near-complete ecological data. The study found that a two-step selection process using ecological data and stratified proportional strategy followed by combined datasets and maximising strategy best represents the whole collection of T. spumosum in the core.

The first genetic maps for subterranean clover (Trifolium subterraneum L.) and comparative genomics with T. pratense L. and Medicago truncatula Gaertn. to identify new molecular markers for breeding

This study reports on the construction of the first genetic maps of subterranean clover (Trifolium subterraneum L.), a diploid, inbreeding annual pasture legume, and alignment of its linkage groups with those of red clover (T. pratense L.) and Medicago truncatula Gaertn. Transferability of red and white clover (T. repens L.) simple sequence repeat (SSR) markers to subterranean clover was observed. A total of 343 SSR loci were mapped into eight subterranean clover linkage groups, with 6–31 loci per linkage group and 27 loci with similar locations between two distinct F2 mapping populations. Phenotypic data obtained for flowering time, content of three isoflavonoids (formononetin, genistein and biochanin A), hardseededness, leaf markings, calyx pigmentation and hairiness of stems were analyzed, together with genotypic data. Genomic intervals influencing each trait were assigned to one to three chromosome regions, accounting for 5.5–59.8% of the phenotypic variance. Syntenic relationships were observed among subterranean clover, red clover and Medicago truncatula genomes. Comparisons of loci shared between the three species indicated that at least two chromosomal regions have undergone duplications in the subterranean clover genome. Candidate genes for isoflavone content were identified using M. truncatula as a reference genome. Synteny-based segmentation observed in Brassicaceae chromosomes helped to account for the apparent segmental-based relationship between the clover genomes, particularly within the subterranean clover lines. The proposed segmental nature of clover genome could account for the extensive variation observed between the parental genotypes, while not preventing production of fertile intercrosses.

Ecogeographical studies identify diversity and potential gaps in the largest germplasm collection of bladder clover (Trifolium spumosum L.)

Plant germplasm collections are important reservoirs of diversity for plant breeders and adding to the knowledge of the diversity among and within germplasm accessions helps plant breeders to use them more efficiently. The annual legume, bladder clover (Trifolium spumosum L.), was the model plant in this study. The hypothesis that ecogeographical variables are important sources of diversity in the germplasm collection of bladder clover and some variables are more important than others has been studied. Three themes have been addressed here: firstly, analysis of the existing passport data to determine a minimum set of ecogeographical descriptors required for capturing ecological aspects of each collection site; secondly, illustration of the ecogeographical regions and subregions using GIS techniques to identify areas with the greatest likelihood of filling gaps in the collection; thirdly, development of an approach for the evaluation of genetic diversity by grouping accessions with a focus on outliers within the collection using principal component and cluster analyses. In this paper the importance of longitude as a geographical factor has been documented and the overall ecogeographical diversity in the germplasm collection of bladder clover has been screened.

Domestication of New Mediterranean Annual Pasture Legumes

Fifteen years ago subterranean clover (Trifolium subterraneum L.) and annual medics (Medicago spp.) dominated annual pasture legume sowings in the Mediterranean-like climate of southern Australia. Since then a number of sustainability and economic challenges to existing farming systems have emerged, exposing shortcomings in these species and a lack of legume biodiversity. A selection program, largely based in Western Australia, with testing sites across southern Australia, has responded to these challenges by domesticating new annual pasture legume species native to the Mediterranean basin to overcome the deficiencies in traditional species (Nichols et al., 2007). Seven new species to agriculture have been commercialised (Ornithopus sativus, Biserrula pelecinus, Trifolium glanduliferum, T. dasyurum, T. spumosum, T. purpureum and Medicago sphaerocarpos), while Lotus ornithopodioides and Melilotus siculus are under evaluation. Traits incorporated include deeper root systems, protection from false breaks (germination-inducing rainfall events followed by death from drought), a range of hardseed levels, acid-soil tolerant root nodule symbioses, tolerance to pests, diseases and salinity and provision of less expensive seed through ease of seed harvesting and processing. The contributions of genetic resources, rhizobiology, pasture ecology and agronomy, plant pathology, physiology, entomology, plant chemistry and animal science have been paramount to this success.

The value of germplasm nurseries in selecting species for field evaluation

This paper documents observations made in irrigated seed multiplication nurseries of novel pasture germplasm to aid in the selection of species for field trials. For the first time it also compares these observations to results from past and concurrent field trials to ascertain the value of observations made in germplasm nurseries in predicting performance in the field. Flowering abundance, seed yield and plant survival over a 2-year period was assessed at four consecutive germplasm nurseries used for seed multiplication between 2002 and 2007. Germplasm included 373 accessions from 67 species and 28 genera of exotic and native perennial forage species, mostly legumes with potential in recharge environments. Classification and regression tree analysis was used to allocate accessions within a species to fewer groups of accessions using the germplasm characteristics. Plant survival and ratings for herbage production in germplasm nurseries were compared to results from field trials for the same species and for the same accessions in order to identify which plant characteristics gave good concordance. Observations from germplasm nurseries were, on the whole, useful in predicting survival of species and accessions in field trials. Ratings for herbage production in germplasm nurseries corresponded poorly with measurements or ratings of herbage production in field trials but, nevertheless, were useful in eliminating some poor-performing germplasm. Several species were identified that had superior survival and seed production in germplasm nurseries, some of which also performed well in field trials, including Dorycnium hirsutum, Kennedia prostrata, Lespedeza juncea var. sericea, Lotononis bainesii and Sanguisorba minor. Western Australian native species had mixed performance in germplasm nurseries but showed promise in field trials, and should be further investigated for their potential in farming systems in southern Australia.

Hotspots and gaps in the world collection of subterranean clover (Trifolium subterraneum L.)

Subterranean clover (Trifolium subterraneum L.) is the most important annual pasture legume in the winter-dominant rainfall areas of Southern Australia. Systematic germplasm collections of subterranean clover from its centre of origin have been made since the 1950s, particularly by Australian scientists, in order to broaden the genetic base of the species. The present study reports on a meta-analysis of the distribution of the world collection of subterranean clovers and their relationships to eco-geographic variables of the collection sites in their native habitat. Diversity hotspots (areas rich in number of accessions and containing a high diversity of sub-species) and also gaps (areas with particular traits un- or under-represented in collections) were identified. This was achieved using a stratified data system to evaluate eco-geographical and agro-morphological data which incorporated three tiers of information for the subterranean clover collection: (1) information from each collection site, including ecological data; (2) information on the phenotypic diversity within each collection site; and (3) plant agro-morphological data from each sample grown under controlled conditions. Correlations were found between some eco-geographic conditions and agronomic performance. These included correlations between latitude and flowering time, mean temperature in winter and winter productivity and precipitation in summer and seed dormancy. The present study concluded that subterranean clover versatility is greater than suggested in the past. The results of the current analysis provide a guide for future collecting missions to specific regions towards areas of maximum diversity (hotspots) and unknown diversity (gaps).

Resistance to clover scorch disease (Kabatiella caulivora) among accessions of purple clover (Trifolium purpureum) and its relationship to the eco-geography of collection sites

Visual ratings of disease reaction to a mixture of races 1 and 2 of clover scorch (Kabatiella caulivora) were conducted on inoculated field plots of 206 accessions of Trifolium purpureum (191 var. purpureum and 15 var. pamphyllicum) collected from the Mediterranean basin and surrounding regions. Disease severity scores of the resistant check, cv. Denmark subterranean clover (T. subterraneum), were clearly differentiated from the susceptible check, cv. Paratta purple clover. Nearly 33% of the accessions were resistant to both races. Resistant plants tended to flower later and originate from higher latitudes, where K. caulivora is more widespread. The results of this investigation led to development of ELECTRA™, the first cultivar of purple clover with resistance to both races of K. caulivora.