José Luis Villaseñor

Biological diversity in the Tehuacán-Cuicatlán Valley, Mexico

A general overview of the biological knowledge of the floristic province of the Tehuacán-Cuicatlán Valley in central-southern Mexico is presented. Floristic and faunistic richness and endemism, as well as uses of the flora are analyzed and discussed for this area, recently declared a biosphere reserve. The analysis shows that, in approximately 10 000 km2 the Tehuacán-Cuicatlán Valley contains between 10 and 11.4% of the Mexican flora. In addition, the valley possesses 365 endemic species that represent 13.9% of its flora. With respect to the fauna diversity, the available information is less comprehensive than for plants. Nevertheless, the study shows that the 11 species of amphibians, 48 species of reptiles, and 91 species of birds recorded for the valley surpasses the diversity found in other dry-lands of the world. In relationship to the mammals of the region, the available data are poor for most of the groups except for bats, for which 24 species have been reported. Regarding the use of the flora, the analysis revealed that 815 species are utilized by the people in the valley. A discussion related to future research activities is also included.

Multiple Pleistocene refugia and Holocene range expansion of an abundant southwestern American desert plant species (Melampodium leucanthum, Asteraceae)

Pleistocene climatic fluctuations had major impacts on desert biota in southwestern North America. During cooler and wetter periods, drought‐adapted species were isolated into refugia, in contrast to expansion of their ranges during the massive aridification in the Holocene. Here, we use Melampodium leucanthum (Asteraceae), a species of the North American desert and semi‐desert regions, to investigate the impact of major aridification in southwestern North America on phylogeography and evolution in a widespread and abundant drought‐adapted plant species. The evidence for three separate Pleistocene refugia at different time levels suggests that this species responded to the Quaternary climatic oscillations in a cyclic manner. In the Holocene, once differentiated lineages came into secondary contact and intermixed, but these range expansions did not follow the eastwardly progressing aridification, but instead occurred independently out of separate Pleistocene refugia. As found in other desert biota, the Continental Divide has acted as a major migration barrier for M. leucanthum since the Pleistocene. Despite being geographically restricted to the eastern part of the species’ distribution, autotetraploids in M. leucanthum originated multiple times and do not form a genetically cohesive group.

Molecular phylogenetic analyses of nuclear and plastid DNA sequences support dysploid and polyploid chromosome number changes and reticulate evolution in the diversification of Melampodium (Millerieae, Asteraceae)

Chromosome evolution (including polyploidy, dysploidy, and structural changes) as well as hybridization and introgression are recognized as important aspects in plant speciation. A suitable group for investigating the evolutionary role of chromosome number changes and reticulation is the medium-sized genus Melampodium (Millerieae, Asteraceae), which contains several chromosome base numbers (x=9, 10, 11, 12, 14) and a number of polyploid species, including putative allopolyploids. A molecular phylogenetic analysis employing both nuclear (ITS) and plastid (matK) DNA sequences, and including all species of the genus, suggests that chromosome base numbers are predictive of evolutionary lineages within Melampodium. Dysploidy, therefore, has clearly been important during evolution of the group. Reticulate evolution is evident with allopolyploids, which prevail over autopolyploids and several of which are confirmed here for the first time, and also (but less often) on the diploid level. Within sect. Melampodium, the complex pattern of bifurcating phylogenetic structure among diploid taxa overlain by reticulate relationships from allopolyploids has non-trivial implications for intrasectional classification.

A proposal for the conservation of the family Cucurbitaceae in Mexico

The pantropical Cucurbitaceae is one of the most important families ofvascular plants. The family includes 118 genera and 825 species, and Mexicois one of its most important centers of diversity, with 34 genera and 141species and subspecific taxa, including 13 cultivated ones. Five genera and 70taxa are endemic to the country. Some Mexican and Latin American wild speciesare close relatives of important crops, and others are employed as food ormedicine. Accordingly, in countries like Mexico the conservation of members ofthis family should be a priority. In this paper a list of members ofCucurbitaceae occurring in Mexico and their distribution patterns at state levelare discussed. Using cluster strategies, the states were classified according totheir floristic similarities. Hotspots of total diversity and endemismsat state level are identified, in order to discuss their role in futureconservation strategies. Iterative methods applied by conservation biology todetermine the best places for conservation are used to identify hierarchicallythe most important states that merit to be considered in this goal. Resultsindicate that implementing conservation strategies in half of the states wouldallow the protection of all the diversity found in the country, especially theendemisms. The combination of the results provided by the different methods isalso discussed as a possibly more efficient way to propose conservationstrategies for important species of the family.

THE PROMISCUOUS AND THE CHASTE: FREQUENT ALLOPOLYPLOID SPECIATION AND ITS GENOMIC CONSEQUENCES IN AMERICAN DAISIES (MELAMPODIUM SECT. MELAMPODIUM; ASTERACEAE)

Polyploidy, an important factor in eukaryotic evolution, is especially abundant in angiosperms, where it often acts in concert with hybridization to produce allopolyploids. The application of molecular phylogenetic techniques has identified the origins of numerous allopolyploids, but little is known on genomic and chromosomal consequences of allopolyploidization, despite their important role in conferring divergence of allopolyploids from their parental species. Here, using several plastid and nuclear sequence markers, we clarify the origin of tetra‐ and hexaploids in a group of American daisies, allowing characterization of genome dynamics in polyploids compared to their diploid ancestors. All polyploid species are allopolyploids. Among the four diploid gene pools, the propensity for allopolyploidization is unevenly distributed phylogenetically with a few species apparently more prone to participate, but the underlying causes remain unclear. Polyploid genomes are characterized by differential loss of ribosomal DNA loci (5S and 35S rDNA), known hotspots of chromosomal evolution, but show genome size additivity, suggesting limited changes beyond those affecting rDNA loci or the presence of processes counterbalancing genome reduction. Patterns of rDNA sequence conversion and provenance of the lost loci are highly idiosyncratic and differ even between allopolyploids of identical parentage, indicating that allopolyploids deriving from the same lower‐ploid parental species can follow different evolutionary trajectories.

Quaternary range dynamics and polyploid evolution in an arid brushland plant species (Melampodium cinereum, Asteraceae)

Pleistocene climatic fluctuations played a principal role for range formation and population history of many biota, including regions not directly affected by glaciations, such as the arid habitats of the southwestern United States and adjacent Mexico. Specifically, drought-adapted species are expected to have persisted during cooler and wetter periods in one or more refugia, resulting in lineage differentiation, from where they reached their current distribution after range expansion in the course of Holocene aridification. Here, we test this hypothesis using Melampodium cinereum (Asteraceae), a morphologically and cytologically variable species of dry brushlands of Texas and adjacent Mexico. In line with the hypothesized presence of several refugia, AFLP data provide strong evidence for the presence of geographically distinct genetic lineages, which, however, only partly agree with current intraspecific taxonomy. Despite multiple origins, tetraploids form a genetically cohesive group. The exclusive occurrence of tetraploids in a range parapatric to that of the diploids likely results from former geographic isolation of cytotypes, lending further support for the presence of Pleistocene refugia. Whereas plastid sequence data show a clear signal for the expected Holocene range and population expansion, they show little geographic structure and high levels of intrapopulational diversity. This may be due to lineage sorting during periods of population separation and/or substantial gene flow among populations via seeds, which has not been sufficient to erode the overall pattern of genetic divergence resulting from geographic isolation.

Vegetation Heterogeneity and Life-Strategy Diversity in the Flora of the Heterogeneous Landscape of Nizanda, Oaxaca, Mexico

We updated the floristic checklist of the Nizanda region, Isthmus of Tehuantepec (southern Mexico), characterized the occurring plant communities based on dominant species, and described the region’s flora according to life form, growth form, growth type, and growth habit spectra. Ten years of botanical exploration, along with surveys in 188 100-m2 samples from different vegetation types, provided the baseline floristic information. Ordination and classification analyses were performed to examine the degree of differentiation between communities. Geographical ranges of all species were used to assess biogeographical relationships of this flora. The inventory includes 920 species (553 genera, 124 families). More than one-third of the families were represented by a single species, whereas the 10 richest families had 43% of the species richness. Dendrograms showing plot classification at three taxonomic levels (species, genus and family) revealed savannah as the most strongly differentiated community amid seven vegetation types. Regarding growth forms, forbs and trees prevailed. Phanerophytes were the most common life form category, whereas herbs and woody plants were the dominant growth types. The largest richness for all taxonomic levels was recorded in the tropical dry forest. The expanded floristic knowledge gained for the Nizanda region provided better criteria to revise the classification scheme of its vegetation. Our preliminary biogeographical analysis illustrates the role of the Isthmus of Tehuantepec as a corridor for thermophilous floras between two oceanic watersheds, and as a natural distributional limit for several Mesoamerican plant species.

Chromosome Numbers, Karyotypes, and Evolution in Melampodium (Asteraceae)

Changes in chromosome number and structure are important contributors to plant evolution and speciation. An excellent system in which to investigate the role of chromosomal changes, especially dysploidy and polyploidy, in species diversification is Melampodium (Millerieae, Asteraceae), which contains a wide variety of chromosome base numbers. We present detailed chromosomal information on numbers and karyotypes obtained from 394 individuals in 111 populations of 39 Melampodium species and interpret it in the context of a recently developed phylogenetic hypothesis. The distribution of chromosome base numbers (including x=14, reported here for the first time) largely agrees with the circumscription of phylogenetic groups, with the exceptions of x=10, which evolved twice independently, and x=11, the putative ancestral chromosome base number still present in a number of distinct lineages. Polyploidy (tetra‐ and hexaploidy) is known from 17 species, 13 of which are exclusively polyploid, and includes autopolyploids and, more frequently, allopolyploids, with a hotspot in sect. Melampodium. Polyploid karyotypes usually do not undergo any obvious structural changes, but the presumably autotetraploid M. dicoelocarpum consistently has a reduced chromosome number (2n=4x−2=46 instead of 2n=4x=48).

An integrated assessment of the vascular plant species of the Americas

The vascular plants of the Americas Botanical exploration in the Americas has a history that stretches back for half a millennium, with knowledge assembled in diverse regional floras and lists. Ulloa Ulloa et al. present a comprehensive and integrated compilation of all known native New World vascular plant species (see the Perspective by Givnish). This compilation, in a publicly available, searchable database, includes 124,993 species—about one-third of the worldwide total. They further present details of the distribution of species across families and genera, the geographical foci of diversity, and the floristic relationships between regions. The rate of plant species discovery in the Americas averages almost 750 annually, so this valuable resource will continue to grow. Science, this issue p. 1614; see also p. 1535 A database of all known New World vascular plants holds almost 125,000 species, with the highest concentration in Ecuador. The cataloging of the vascular plants of the Americas has a centuries-long history, but it is only in recent decades that an overview of the entire flora has become possible. We present an integrated assessment of all known native species of vascular plants in the Americas. Twelve regional and national checklists, prepared over the past 25 years and including two large ongoing flora projects, were merged into a single list. Our publicly searchable checklist includes 124,993 species, 6227 genera, and 355 families, which correspond to 33% of the 383,671 vascular plant species known worldwide. In the past 25 years, the rate at which new species descriptions are added has averaged 744 annually for the Americas, and we can expect the total to reach about 150,000.

Distributional patterns and conservation of species of Asteraceae (asters etc.) endemic to eastern Mexico: a panbiogeographical approach

Abstract Biogeographical patterns of distribution of 74 species of Asteraceae that inhabit mainly the temperate forests of eastern Mexico were studied usingtrack analysis. Five generalized tracks were identified and conservation areas proposed based on degree of complexity of floristic patterns (Luna et al., 1999) and biotic richness. The northern part of the Sierra Madre Oriental harbours a high concentration of narrowly restricted species. In this area there are four track nodes that coincide with the Mexican priority terrestrial regions (RTPs) proposed by the Comisión Nacional para el Conocimiento y Uso de la Biodiversidad (CONABIO) and with previously identified areas of importance for bird conservation (AICAs). Two more nodes are found in the central and southern part of the Sierra. With the gathered information, we propose particular selected species of Asteraceae to be included in the Mexican areas of endangered species (NOM).