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Haplopappus gracilis (n = 2), Haplopappus ravenii (n = 4), and Haplopappus wigginsii (n = 4) are isolated by F1 hybrid sterility due mainly to translocation heterozygosity. There is no evidence that this can be overcome at the diploid level so that introgression can occur among them. They are also separated geographically, but occasional populations of H. gracilis and H. ravenii may be brought together along roadways to form sterile hybrids. There were no statistically significant differences in nuclear DNA content among the same or structurally different aneuploid n = 2 and n = 3 chromosome races or ecotypes of H. gracilis. Some of the H. gracilis races were not significantly different from one race of the ancestral H. ravenii, and these samples of both species were from plants growing on poor soils in contrast to accessions from normal habitats. How much and which classes of DNA in these species are subject to changes induced by environmental effects is not known. There were no correlations between DNA amounts and altitude, latitude, and longitude. H. wigginsii had a greater amount of DNA per nucleus than either H. ravenii or H. gracilis, and its increased DNA content may reflect a more rapid accumulation of noncoding sequences due to facultative self-compatibility not found in the other two species.

Fraser fir (Abies fraseri (Pursh) Poir.) is an endemic species consisting of six major disjunct populations in the Appalachian Mountains, U.S.A. Nuclear DNA content was measured with laser flow cytometry to determine if genome size differences could be detected among the disjunct populations of Fraser fir and its close relatives, balsam fir (Abies balsamea (L.) Mill.) and Canaan fir (A. balsamea var. phanerolepsis Fern.). The mean DNA content for Fraser fir was 17.2 pg/C, which was similar to the two related fir species. There were no significant differences among disjunct Fraser-fir populations. Mean DNA content differences for fir species in the southern Appalachian Mountains were similar even with speciation events (7000 B.P.) and subsequent population isolation. In the absence of polyploidy or large chromosomal rearrangements, genome size changes in conifers occur on a broad evolutionary time scale.Key words: conifers, gymnosperm, C-values, nuclear genome, Abies fraseri.Le sapin de Fraser (Abies fraseri (Pursh) Poir.) est une espèce endémique comportant six populations disjointes dans la chaîne montagneuse des Appalaches, aux Etats-Unis. Afin de déterminer si on peut déceler des différences dans la dimension du génome entre les populations disjointes du sapin de Fraser et ses proches parents, soient le sapin baumier (Abies balsamea (L.) Mill.) et le sapin de Canaan (A. balsamea var. phanerolepsis Fern.), les auteurs ont mesuré leurs contenus en acides nucléiques à l'aide de la cytométrie en flux au laser. La teneur moyenne en ADN chez le sapin de Fraser est de 17.2 pg/C, qui est similaire à celle des deux espèces de sapin voisines. Il n'y a pas de différence entre les populations disjointes du sapin de Fraser. Les différences de teneur moyenne en ADN chez les espèces de sapin du sud des montagnes appalachiennes sont semblables en dépit d'événements de spéciation (7000 A.P.) et l'isolation subséquente des populations. En absence de polyploïdie ou de larges réarrangements chromosomiques, les changements de dimension du génome chez les conifères s'effectue sur une large échelle temporelle d'évolution.Mots clés : conifères, gymnosperme, valeur-C, génome nucléique, Abies fraseri.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]