Large numbers of entries are now lodged in many of the world's germ-plasm collections of crop and pasture plants. This abundance of material, assembled to guard against its irretrievable loss, has intensified the problems of how best to conserve it and how to use it in plant breeding. Core collections have a major role to play in solving these problems. The core is composed of about 10% of the total collection, chosen to represent as much as possible of the diversity in the collection. The selection of the core entries should use the available data on the geographic origin, the genetic characteristics, and the possible value to breeders and other users of each accession in the collection. Stratified sampling from groups of accessions, in logarithmic or absolute proportion to the group size, is the best strategy. The core entries should be kept separately and not in one bulk population. The composition of the core should be adjusted as new material is included, or better data are obtained. The remaining accessions in the collection form the reserve, which should be conserved as secondary sources.Key words: germ-plasm collections, sampling strategies, Glycine tomentella, Hordeum vulgare.

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Published data on allele frequencies at isozyme loci in inbreeding and outbreeding plant species were analyzed to examine intraspecific variation in gene diversity and effective population size (Ne). Compared with outbreeders, inbreeding species showed markedly greater variation among populations in average values of Nei's gene diversity statistic. Effective population size was estimated by assuming that the variation observed at isozyme loci is selectively neutral. Inbieeding species showed greater levels of variation in Ne than did outbreoders, although the upper range ofNe was similar in the two classes of species. The results suggest that there may be considerable genetic variation and potential for evolutionary change in some but not all populations of inbreeders. Moreover, these findings are important with respect to the conservation of genetic resources. In particular, that the amount of intraspecific variation in population genetic diversity and Ne differs between inbreeding and outbreeding species should be taken into account in sampling efforts designed to optimize the diversity of germplasm collections.Studies ofgenetic variation in plants have typically used Nei's genetic diversity statistics or Wright's F statistics as tools for describing the extent of differentiation among populations (1)(2)(3). While informative, these statistics do not provide direct information about the amount of among-population variation in levels of polymorphism and gene diversity, a feature that can be of key importance in efforts to conserve the genetic resources of a species. In this regard, a survey of isozyme variation in several plant populations conducted by Brown (4) suggested that inbreeding species will tend to show greater variation among populations in the level of genetic diversity compared with outbreeding ones. Brown (4) proposed that, in inbreeders, among-population variation in the average value of Nei's gene diversity statistic will often exhibit an L-shaped or bimodal distribution. Published data for the predominantly self-pollinating plants Hordeum spontaneum and Lycopersicon pimpinellifolium supported this view (4).In this investigation, we analyze additional published isozyme data from inbreeding and outbreeding plant species in order to explore in detail the nature and implications of variation in genetic diversity among populations of inbreeding and outbreeding species. This is done by examining the level of among-population variation in gene diversity. We have also asked what the observed patterns of variation in diversity imply about the amount of variation in effective population size in species with contrasting breeding systems.MATERIALS AND METHODS Published tables of allozyme frequency data were used in all analyses described below. These data sets were selected according to the following criteria: (i) the mating system of the species in question was known either through progeny testing of open-pollinated seed parents (5), from analysis of genotype frequencies (6), or based on the obs...

Varietal data from 27 crop species from five continents were drawn together to determine overall trends in crop varietal diversity on farm. Measurements of richness, evenness, and divergence showed that considerable crop genetic diversity continues to be maintained on farm, in the form of traditional crop varieties. Major staples had higher richness and evenness than nonstaples. Variety richness for clonal species was much higher than that of other breeding systems. A close linear relationship between traditional variety richness and evenness (both transformed), empirically derived from data spanning a wide range of crops and countries, was found both at household and community levels. Fitting a neutral “function” to traditional variety diversity relationships, comparable to a species abundance distribution of “neutral ecology,” provided a benchmark to assess the standing diversity on farm. In some cases, high dominance occurred, with much of the variety richness held at low frequencies. This suggested that diversity may be maintained as an insurance to meet future environmental changes or social and economic needs. In other cases, a more even frequency distribution of varieties was found, possibly implying that farmers are selecting varieties to service a diversity of current needs and purposes. Divergence estimates, measured as the proportion of community evenness displayed among farmers, underscore the importance of a large number of small farms adopting distinctly diverse varietal strategies as a major force that maintains crop genetic diversity on farm.

The exploration, conservation and use of the genetic resources of plants is a contemporary issue which requires a multidisciplinary approach. Here the role of population genetic data, particularly those derived from electrophoretic analysis of protein variation, is reviewed. Measures of the geographic structure of genetic variation are used to check on sampling theory. Current estimates justify the contention that alleles which have a highly localised distribution, yet are in high frequency in some neighbourhoods, represent a substantial fraction of the variation. This class, which is the most important class in the framing of sampling strategies, accounts for about 20-30% of variants found in 12 plant species. The importance of documenting possible coadapted complexes and gene-environment relationships is discussed. Furthermore, the genetic structure of natural populations of crop relatives might suggest the best structure to use in the breeding of crops for reduced vulnerability to pest and disease attack, or for adaptation to inferior environments. The studies reported to date show that whilst monomorphic natural populations do occur, particularly in inbreeding colonisers, or at the extreme margins of the distribution, polymorphism seems to be the more common mode. It is stressed here that the genetic resources of the wild relatives of crop plants should be systematically evaluated. These sources will supplement, and might even rival, the primitive land races in their effectiveness in breeding programmes. We may look forward to a wider application of gel electrophoresis in the evaluation of plant genetic resources because this technique is currently the best available for detecting genetic differences close to the DNA level on samples of reasonable size.

Wild crop relatives are an important source of genetic variation for improving domesticated species. Given limited resources, methods for maximizing the genetic diversity of collections of wild relatives are needed to help spread protection over a larger number of populations and species. Simulations were conducted to investigate the optimal strategy of sampling materials from populations of wild relatives, with the objective of maximizing the number of alleles (allelic richness) in collections of fixed size. Two methods, based on assessing populations for variation at marker loci (e.g., allozymes, restriction fragment length polymorphisms), were developed and compared with several methods that are not dependent on markers. Marker-assisted methods yielded higher overall allelic richness in the simulated collections, and they were particularly effective in conserving geographically localized alleles, the class of alleles that is most subject to loss.Decisions in conservation biology may be based on demographic or genetic criteria or both (1). Demography often takes precedence when populations face immediate threats (2), but the long-term viability of species requires genetic variation (3). In domesticated species the conservation of single-locus variation is of special interest, as disease resistance and other economically important characteristics are often inherited in simple Mendelian fashion (4,5). Genes introduced from the wild relatives of crops constitute an important source of single-locus variation for the improvement of domesticated species (6-8). The diversity of alleles at single loci (allelic richness) in wild relatives is particularly vulnerable to loss due to reduction in population size (9, 10). The problem is compounded by the fact that most crop relatives are found only in nature, and many such species and populations are increasingly threatened by habitat reduction (8). Moreover, there are many potentially useful populations of wild relatives, yet for practical purposes only a fraction of all such material can be afforded protection or maintenance in gene banks or in nature reserves. In addition, wild relatives are often geographically wide ranging, making it costly to collect representative samples of these materials. By maximizing genetic diversity in germ-plasm collections of fixed size, resources available for conservation of crop biodiversity can be allocated to a larger number of species.This paper is concerned with how genetic markers (allozymes, restriction fragment length polymorphisms, etc.), which have been successfully employed in many other applied aspects of biology and medicine, can be used to help construct collections of wild crop relatives having maximal allelic richness. Because of their increasing importance in germ-plasm conservation we focus on the core collection (11, 12 MATERIALS AND METHODSTwo marker-assisted strategies for constructing core collections were developed. Each delineates how accessions are to be selected from a larger collection that has been div...

Aim To determine whether latitudinal and longitudinal gradients in seed mass are related to variation in climatic features including temperature, solar radiation and rainfall.Location Australia.Methods Seed mass was estimated from over 1600 provenances covering the latitudinal and longitudinal extents of 34 perennial Glycine taxa in Australia. Climatic data were obtained from ANUCLIM 5.1 for collection locations based on long-term meteorological records across Australia. These climatic data were subject to principal components analysis to extract three components as climatic indices. Generalized linear models were used in three separate sets of analyses to evaluate whether seed mass-latitude and seed mass-longitude relationships persisted after taking climatic variation into account. First, relationships were examined across species in analyses that did not explicitly consider phylogenetic relationships. Secondly, phylogenetic regressions were performed to examine patterns of correlated evolutionary change throughout the Glycine phylogeny. Within-species analysis was also performed to examine consistency across different taxonomic levels.Results Geographical variation in seed mass among species was related primarily to temperature and solar radiation, while rainfall was much less influential upon seed mass. Partialing out the influence of temperature and solar radiation in models resulted in the disappearance of significant seed masslatitude and seed mass-longitude relationships. Patterns within species were generally consistent with patterns among species. However, in several species, factors additional to these climatic variables may contribute to the origin and maintenance of geographical gradients in seed mass, as significant seed masslatitude and seed mass-longitude relationships remained after controlling for the influence of climatic variables.Main conclusions Our empirical results support the hypotheses that (1) seed mass is larger at low latitudes and in the interior of the Australian continent due to increased metabolic costs at high temperatures, and that (2) higher levels of solar radiation result in an increase in the availability of photosynthate, which in turn leads to an increase in biomass for the production of large seeds. In effect, our findings show that greater energy is available precisely where needed, that is, where high temperatures require large seed mass on the basis of metabolic requirements.

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