Isozyme polymorphism and geographic differentiation in a collection of French perennial ryegrass populations

Abstract

A sample of 60 natural populations of perennial ryegrass from France has been studied for allelic variation at 7 polymorphic enzyme loci . Population genetic statistics are of the same magnitude than those previously reported for other outbreeding, short-lived perennial species (P = 64%, A = 2 .75, H = 0 .270) .Genotype frequencies at most collection sites do not deviate significantly from Hardy-Weinberg expectations, with however a slight deficit of heterozygotes which may be accounted for by a Wahlund effect. Gene diversity is mainly explained by the within population component . The between population differentiation F s, averaged on 4 loci is only 0 .054, which accounts for only 6% of the whole diversity .When mapped, most allele frequencies do not show any special structure . Only five alleles present a clinal trend from North to South . These 5 alleles are probably related to some climatic factors such as average temperature or potential evapotranspiration . The causal hypotheses about the low level of betweenpopulation differentiation and spatial structure are discussed with reference to the literature .The consequences of the found population structure for sampling and conservation strategies of natural populations for genetic resources are presented and discussed .

Geographical patterns of isozyme variation in Mediterranean populations of perennial ryegrass

“…In fact, although the results do not differ significantly between populations (because only a small number of populations were sampled from France and Sardinia), Table 2 indicates that on average French populations show lower diversity indices than Corsican accessions. These results are consistent with those found by Charmet et al (1993), from a sample of 60 wild populations collected throughout France. This could be explained by the fact that the centre of origin of the genus Lolium is nearer to the Mediterranean regions of Corsica and Italy than to France.…”

“…The hypothesis of the introduction of perennial ryegrass from Italy to Corsica is reinforced by the phenogram (Fig. 2) Finally, Sardinian accessions appear very different from continental material (Table 4) Sardinian material also presents some rare alleles such as PGM-c or MDH-a According to recent results (Charmet & Balfourier, 1993), we know that these alleles are more frequent in the other cross-pollinated species, Lolium multiflorum and Lolium rigidum. The presence of such rare alleles in Sardinia may result from introgression between these species and L. perenne (Terell, 1968), hence providing an explanation of the diversity of the four populations studied here, compared with the others.…”

Isozyme polymorphism in a collection of Spanish and French perennial ryegrass populations

“…Our diversity indices are very similar to those reported by Hayward (1985) in 40 natural populations from Britain (A = 3.09, H = 0.372) on five loci, by Oliveira and Charmet (1988) (1977) for L perenne cultivars, Oliveira and Charmet (1988) for L perenne Galician populations, Charmet et al (1993) for French perennial ryegrass populations and Loos (1994) for European populations and cultivars of L perenne L. This low value leads to the conclusion that collected populations are nearly panmictic.…”

“…Survey of isozyme variation in natural populations of forage grasses have already been reported for ryegrass (Hayward, 1985;Arcioni et al, 1988;Oliveira and Charmet, 1988;Charmet et al, 1993;Balfourier and Charmet, 1994;Charmet and Balfourier, 1994;Loos, 1994). Variation at isozyme loci is generally considered as selectively neutral (Kimura, 1983).…”

Conservation of allelic multiplicity and genotypic frequency by pooling wild populations of perennial ryegrass

“…We might be surprised by the similarity of the populations in their allelic frequencies despite the fact that they have been allocated to different groups by ecogeographical origin and cluster analysis on agronomic traits. In fact, for an allogamous species like L. perenne, there is little differentiation between wild populations, most of the differentiation being within populations (Charmet et al, 1993). A practical consequence of this low differentiation between populations could be that a small number of populations would be sufficient to preserve most of the genetic diversity.…”