Cytoevolution of taxa in Acacia and Prosopis (Mimosaceae)

Somatic chromosome numbers were reported for 47 accessions of Acacia (Tourn.) Miller, representing 23 species including A. albida (syn. Faidherbia albida), together with 19 accessions (11 species) of Prosopis (L.) Burkart. Microphotographs of somatic metaphases of 12 accessions and six representative idiograms are shown. Somatic numbers of different accessions of a species can be different. Somatic numbers of accessions of subgenus Acacia were predominantly 52; a few had 2n = 78 or 104. However, A. sieberana had 2n = 26 and A. nubica 2n = 56. Except for A, polycantha, which had 2n = 52, somatic numbers of subgenus Aculeiferum were invariably 26. Of the six species of subgenus Heterophyllum studied, five had 26 somatic chromosomes and one (A. holoceraceae) had 52. All the four accessions of A. albida had 2n = 26. Out of the 19 Prosopis accessions investigated, five had 56 somatic chromosomes and 14 accessions had 28. In Acacia, subgeneric and specific differentiations are probably caused by structural changes in the chromosomes, whereas differences in the degree of euploidy appear to create infraspecies. Subgenus Aculeiferum is a possible progenitor of all other taxa of Acacia and Prosopis. The study suggests that Prosopis has diverged later in geological times from a diploid ancestor (2n = 26) of Aculeiferum through centric fission of the second-largest chromosome pair.