Juan H. Hunziker

Hybridization and genetic variation of Argentine species of Prosopis

Abstract The taxonomic division of the Prosopis genus, its geographic distribution, and the factors that limit it are briefly considered. All of these taxa except one are diploid (2 n = 28). Several species of sect. Algarobia are known to be self-incompatible and cross-fertilizing. The data on genetic variation concerning the percentage of polymorphic loci ( P ) and the expected mean of heterozygotes per locus ( H ) shows that arboreal species belonging to sect. Algarobia possess more genetic variation ( P = 38−50%, H = 0.132−0.223 ) than the shrubby species of sect. Strombocarpa ( P = 8.69−17.39%, H = 0.022−0.063 ). Within the first group, the ‘vinal’ ( P. ruscifolia ), an aggressive woody colonizer of northeastern Argentina and southern Paraguay, is the species that possesses the lowest measures of genetic variation ( P = 38%, H = 0.132 ± 0.044 ). It may be that the adaptive strategy of this noxious weed involves less genetic variability than that possessed by its congeners along with more phenotypic plasticity and wider ecological tolerance. The possibility that the species of Strombocarpa may be self-compatible is discussed; self-fertilization could explain the lower genetic variation displayed by their populations. Interspecific hybridization is frequent between species of Algarobia , and hybrids in general have regular meiotic pairing. No hybrids between Algarobia and Strombocarpa have been described so far. This and the strong differences in the mobility of certain enzymes between species belonging to one or the other section indicates that it would be more appropriate to raise the sections at least to the level of subgenera. Chromatography of phenolic compounds has been used with success by several authors for the characterization of species and identification of interspecific hybrids. A list of all natural Prosopis hybrids of section Algarobia so far cited is given (Table 1). The promising Argentine species for cultivation are P. alba, P. nigra, P. chilensis, P. flexuosa and P. hassleri and their most important uses are mentioned. Finally, the abusive exploitation of the ‘algarrobo’ woods in Argentina and the urgent need for genetic conservation of the most valuable and promising species is pointed out.

Nuclear DNA variation in diploid and polyploid taxa of Larrea (Zygophyllaceae)

A study of nuclear DNA content was made in telophase nuclei (2C) of the root apex of germinating seed in nine populations of the following species and cytotypes of Larrea: L. nitida (2x), L. divaricata (2x), L. cuneifolia (4x) and L. tridentata (2x, 4x, 6x). There were no significant differences in DNA content per basic monoploid genome among the diploid taxa nor between the latter and the tetraploid, among tetraploids or between tetraploids and the hexaploid. On the other hand, the difference between means was significant when all diploids were compared with the hexaploid cytotype. These results would indicate:(1) Speciation at the diploid level in Larrea has not produced great differences in DNA content per basic genome. This is in contrast with the related genus Bulnesia.(2) In Larrea there is a slight diminution in DNA content per basic genome when there is an increase in ploidy level.(3) Species of Larrea, Bulnesia and Pintoa (Zygophyllaceae) that inhabit the most arid environments are the ones possessing the highest DNA content.(4) This increase is due to an increment in ploidy level in Larrea and an augment of intrachromosomal DNA in Bulnesia and Pintoa.

Hybridization between South American Jarilla and North American diploid creosotebush (Larrea, Zygophyllaceae)

Different strains of the two diploidLarrea speciesL. divaricata (S. America) andL. tridentata (N. America), are compared morphologically, cytologically, and in their seed proteins. There are minor differences, also in chiasma frequencies. Experimental F1 hybrids have near-normal meiosis but are semisterile. The parental species, therefore, biologically may be considered “allopatric semispecies”.

Chromosome studies on the Bambusoideae (Gramineae)

Meiotic chromosome numbers and observations were made in eleven species belonging to nine different genera of Bambusoideae. The numbers include counts in eight species that have never been investigated cytologically:Arundinaria aff.amabilis (n=24), Chusquea bambusoides (n=ca. 20),Cryptochloa dressleri (n=11), Olyra ciliatifolia (n=11), Sucrea monophylla (n=11), Raddia brasiliensis (n=11), Strephium distichophyllum (n=11), andPariana aurita (n=22). The generaStrephium andSucrea have not been studied previously with respect to their chromosomes. The significance of these results are discussed in relation to the over 140 published counts in the subfamily, which indicates a preponderance of polyploidy in the woody members, or bamboos, but not in their herbaceous allies. Aneuploidy is rare in the former but frequent in the latter. It is postulated that the herbaceous bambusoids, which flower every year and produce recombinants regularly and in short-time cycles, could have tolerated successive reductions of the linkage groups and their genetic flexibility. These changes would not have been favored in the bamboos, which have more genetic constancy due to the long lapse (often 20–120 years) between flowering generations.

Species relationships in Bulnesia as shown by seed protein electrophoresis

Abstract The seed proteins of seven species of Bulnesia were studied by polyacrylamide electrophoresis. Some of the bands are characteristic and constant “markers” of each species; these allow the unequivocal identification of their electrophoregrams. In total 84 different bands were identified. These were treated numerically by cluster analysis. There were no constant differences between geographic races of B. arborea from Colombia and Venezuela. The electrophoregram of B. carrapo shows differences with that of B. arborea giving support to the idea that both taxa are separate allopatric species. The species pair B. foliosa-B. schickendantzii present the most similar electrophoregrams; this determines a short taxonomic distance between them in the phenogram. The Prim network shows the supposedly more primitive species ( B. arborea, B. carrapo and B. bonariensis ) well separated from the more advanced group ( B. schickendantzii, B. foliosa and B. retama ). B. sarmientoi , however, appears as rather distant and unrelated from all other taxa. In general, the results from protein electrophoresis agree with results from a previous numerical study based on 43 morphological characters.

Chromosome number inHalophytum ameghinoi (Halophytaceae)

The chromsome number and karyotype ofHalophytum ameghinoi (Speg.) Speg. was found to be 2n = 24 and 12m+10sm+2t, respectively. Meiosis was regular and 12 bivalents were observed. The relationships of this monotypic family are briefly discussed.

Isozyme variation inBulnesia retama, B. schickendantzii andB. foliosa (Zygophyllaceae)

The genetic variation between two allopatric populations ofBulnesia retama and that ofB. schickendantzii andB. foliosa was investigated. The Peruvian population ofB. retama showed low values of P (0.117) and He (0.057) compared to those in the Argentine population with P = 0.59 and He = 0.269;B. schickendantzii showed P = 0.54 andB. foliosa P = 0.17. Genetic identity between the latter was 0.836 and between the allopatric populations ofB. retama was 0.89; the Peruvian population had reduced allelic variation per locus (A = 1.176) in comparison to the Argentine population (A = 1.955). The values of A, P, He andWrights fixation indices suggest that the Peruvian population could have originated from a single or very few migrants from southern latitudes (founder effect).

Some historical aspects of plant cytogenetics in Argentina and Uruguay

A brief account is given of the origin and development of plant cytogenetics in Argentina and Uruguay, along with some of the factors that hampered the development of this area.