Juan H. Hunziker

Bio: Juan H. Hunziker is an academic researcher from Facultad de Ciencias Exactas y Naturales. The author has contributed to research in topics: Bulnesia & Larrea. The author has an hindex of 10, co-authored 25 publications receiving 403 citations.

Species Disjunctions in Larrea: Evidence From Morphology, Cytogenetics, Phenolic Compounds, and Seed Albumins

Abstract: Larrea divaricata, the "creosote bush," is often regarded as the most drought tolerant higher plant in North America (Barbour, 1969; Morello, 1955). It is probably a single species having an enormous disjunct distribution. In North America it extends from the State of Nevada to the State of Hidalgo in Mexico, where it is called "gobernadora" on account of its dominance in the natural vegetation (Fig. 1). In South America it occurs in several isolated places in Peru (Ica, Nazca, Chuquibamba, Moquegua), in Bolivia (Tarija), and in Chile (Atacama, Concepcion). In Argentina it is called "jarilla," and it extends from Salta in the North to North Patagonia (Chubut) (Fig. 2). As pointed out by Barbour the disjunction reaches 36 degrees latitude, that is, nearly 4,000 km (Barbour, 1969). The question that immediately arises, when considering such an extense distribution is whether we are dealing with a single species or at least two vicariant species: Larrea tridentata (North America) and L. divaricata (South America). How much have these populations diverged since they became separated? Typical Larrea divaricata was described from Mendoza, Argentina, South America, and the North American taxon has been named L. tridentata. Morphologically, they can easily be separated by the form of the stipules, the North American populations having acute while the South American ones have obtuse stipules. They could be distinguished, therefore, as different subspecies as done recently by Felger and Lowe (1970) who recognize L. divaricata Cav. subsp. divaricata for the South American subspecies and its ecotypes and subsp. tridentata (Sesse & Moc. ex DC.) Felger & Lowe for the North American subspecies and its ecotypes.

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

Abstract: 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.

Genome downsizing in polyploid plants

Abstract: All else being equal, polyploids are expected to have larger C-values (amount of DNA in the unreplicated gametic nucleus) than their diploid progenitors, increasing in direct proportion with ploidy. This expectation is observed in some polyploid series, especially those newly formed, but there are examples suggesting that C-values in particular polyploids are less than expected. The availability of the Angiosperm DNA C-values database (http:// www.rbgkew.org.uk/cvavhomepage.html) has allowed this question to be addressed across a broad range of angiosperms and has revealed striking results deviating from expectation: (i) mean 1C DNA amount did not increase in direct proportion with ploidy, and (ii) mean DNA amount per basic genome (calculated by dividing the 2C value by ploidy) tended to decrease with increasing ploidy. These results suggest that loss of DNA following polyploid formation, or genome downsizing, may be a widespread phenomenon of considerable biological significance. Recent advances in our understanding of the molecular events that take place following polyploid formation together with new data on how DNA amounts can both increase and decrease provide some insights into how genome downsizing may take place. The nature of the evolutionary forces that may be driving DNA loss are also discussed.

Physiological Ecology of North American Desert Plants

Abstract: This book begins with the physical and biological characterization of the four North American deserts and a description of the primary adaptations of plants to environmental stress. In the following chapters the authors present case studies of key species representing dominant growth forms of the North American deserts, and provide an up-to-date and comprehensive review of the major patterns of adaptations in desert plants. One chapter is devoted to several important exotic plants that have invaded North American deserts. The book ends with a synthesis of the adaptations and resource requirements of North American desert plants. Further, it addresses how desert plants may respond to global climate change.

Polyploidy, hybridization, and the invasion of new habitats.

Abstract: Experiments are described showing that most artificial autopolyploids derived from native or introduced perennial grass species from California are far inferior in behavior under field conditions than their diploid ancestors In a single species, Ehrharta erecta in two out of 22 localities, the autotetraploid maintained itself for, respectively, 19 and 39 years, but remained either in the exact locality of planting or in nearby localities having very similar ecological conditions The control diploids, direct descendants of the progenitors of the autopolyploids, spread more widely and evolved more variation with respect to growth habit among each progeny These results, along with other evidence derived from several literature sources, strengthen the hypothesis that successful polyploids among natural populations are usually or almost always the result of increased heterozygosity accompanying either interracial or interspecific hybridization The hypothesis that polyploids succeed because of their greater tolerance of severe ecological or climatic conditions is again rejected, and that which postulates secondary contacts between previously isolated populations as the principal cause for their high frequencies in some groups of angiosperms is favored The unusually high frequency of polyploids in the Gramineae is attributed to the probable fact that habitats to which they are best adapted have changed in extent and position repeatedly during the geological periods since the initial evolution of the family The family Gramineae contains higher percentages of species and races or cytotypes of polyploid origin than any other large family of angiosperms More than 80% of its species have undergone polyploidy some time during their evolutionary history Polyploidy is expressed by four different kinds of numerical series, as fol-

Variation in genomic form in plants and its ecological implications

Abstract: Summary The gross form of the nuclear genome varies greatly among plant species in both anatomy and genetic organization. Chromosome number (ft) ranges from 2 to over 600, and ploidy from 1 to over 20. The amount of DNA in the unreplicated haplophase genome (the 1C value) differs by more than 2500-fold among angiosperms. Although it has been questioned since the 1930s whether such variation is of adaptive significance and whether it is related, perhaps causally, with environmental factors, no direct or causal links have yet been found. However, variation in DNA C-value has far-reaching biological consequences and can be of considerable adaptive and hence ecological significance. Strikingly precise interspecific relationships exist between DNA C-value and many diverse phenotypic characters at the cellular level, and DNA can affect the phenotype in two ways, firstly by expression of its genie content and, secondly, by the biophysical effects of its mass and volume, the latter defined as nucleotypic effects. Nucleotypic variation in DNA C-value sets absolute limits to both the minimum size and mass of the basic unit of plant anatomy (i.e. the cell) and the minumum time needed to produce a similar cell with newly synthesized organic molecules. Moreover, in complex multicellular vascular plants, such effects at successive cell cycles are additive, so that DNA C-value influences many characters, including growth rate, seed weight, minimum generation time and type of life-cycle. Thus, the nucleotype profoundly affects where, when and how plants grow. Selection for a particular genomic form acting on its spatial or temporal consequences may occur at various levels ranging from the cell to the whole organism and may operate throughout the life-cycle or at just one stage. DNA C-value is often indirectly related to environmental factors which determine time-limited environments via selection acting on the temporal phenotypic consequences of nucleotypic variation. However, in the case of radio-sensitivity, selection for a low DNA C-value may act directly on the nucleotype itself, as the size of the nuclear DNA target directly affects the ability of the plant to survive.