Journal Article

Relationships of the Woody <i>Medicago</i> Species (Section <i>Dendrotelis</i>) Assessed by Molecular Cytogenetic Analyses

Marcela Rosato, Mercedes Castro and Josep A. Rosselló

in Annals of Botany

Published on behalf of The Annals of Botany Company

Volume 102, issue 1, pages 15-22
Published in print July 2008 | ISSN: 0305-7364
Published online April 2008 | e-ISSN: 1095-8290 | DOI: https://dx.doi.org/10.1093/aob/mcn055
Relationships of the Woody Medicago Species (Section Dendrotelis) Assessed by Molecular Cytogenetic Analyses

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Background and Aims

The organization of rDNA genes in the woody medic species from the agronomically important Medicago section Dendrotelis was analysed to gain insight into their taxonomic relationships, to assess the levels of infraspecific variation concerning ribosomal loci in a restricted and fragmented insular species (M. citrina) and to assess the nature of its polyploidy.

Methods

Fluorescence in situ hybridization (FISH) was used for physical mapping of 5S and 45S ribosomal DNA genes in the three species of section Dendrotelis (M. arborea, M. citrina, M. strasseri) and the related M. marina from section Medicago. Genomic in situ hybridization (GISH) was used to assess the genomic relationships of the polyploid M. citrina with the putatively related species from section Dendrotelis.

Key Results

The diploid (2n = 16) M. marina has a single 45S and two 5S rDNA loci, a pattern usually detected in previous studies of Medicago diploid species. However, polyploid species from section Dendrotelis depart from expectations. The tetraploid species (2n = 32) M. arborea and M. strasseri have one 45S rDNA locus and two 5S rDNA loci, whereas in the hexaploid (2n = 48) M. citrina four 45S rDNA and five 5S rDNA loci have been detected. No single chromosome of M. citrina was uniformly labelled after using genomic probes from M. arborea and M. strasseri. Instead, cross-hybridization signals in M. citrina were restricted to terminal chromosome arms and NOR regions.

Conclusions

FISH results support the close taxonomic interrelationship between M. arborea and M. strasseri. In these tetraploid species, NOR loci have experienced a diploidization event through physical loss of sequences, a cytogenetic feature so far not reported in other species of the genus. The high number of rDNA loci and GISH results support the specific status for the hexaploid M. citrina, and it is suggested that this species is not an autopolyploid derivative of M. arborea or M. strasseri. Further, molecular cytogenetic data do not suggest the hypothesis that M. arborea and M. strasseri were involved in the origin of M. citrina. FISH mapping can be used as an efficient tool to determine the genomic contribution of M. citrina in somatic hybrids with other medic species.

Keywords: Medicago arborea; M. citrina; M. strasseri; rRNA genes; 18S-5·8S-25-S; 5S; FISH mapping; GISH; polyploidy

Journal Article.  5320 words.  Illustrated.

Subjects: Ecology and Conservation ; Evolutionary Biology ; Plant Sciences and Forestry

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