Gabriel H. Rua

A preliminary approach to the phylogeny of the genus Paspalum (Poaceae)

The present work intends to clarify the phylogenetic relationships among the species of Paspalum L. belonging to the informal groups Notata/Linearia and Dilatata, and to raise some preliminary hypotheses on the phylogeny of the genus as a whole. A combined dataset including morphological and molecular characters was used to analyze 28 species of Paspalum plus some representatives of related genera of the tribe Paniceae. Analyses were performed using both parsimony and maximum likelihood. The monophyly of Paspalum is not supported nor contradicted. The circumscription of informal groups of Paspalum is discussed, as well as the cladistic treatment of allopolyploid taxa, especially those comprising the Dilatata group. The relationships of members of the Dilatata with their putative progenitors is confirmed, but the monophyly of the group as a whole is not. A close relationship between P. dilatatum Poir. and P. lividum Trin. ex Schltdl. is shown. Our analysis is consistent with the monophyly of a group comprising Notata+Linearia, with a monophyletic Notata group nested within it. The delimitation of the core Notata is proposed by including P. conduplicatum Canto-Dorow, Valls and Longhi-Wagner, P. notatum Flüggé, P. minus E. Fourn., P. pumilum Nees and P. subciliatum Chase.

A phylogenetic analysis of the genus Paspalum (Poaceae) based on cpDNA and morphology

With about 350 species, Paspalum is one of the richest genera within the Poaceae. Its species inhabit ecologically diverse areas along the Americas and they are largely responsible for the biodiversity of grassland ecosystems in South America. Despite its size and relevance, no phylogeny of the genus as a whole is currently available and infrageneric relationships remain uncertain. Many Paspalum species consist of sexual-diploid and apomictic-polyploid cytotypes, and several have arisen through hybridization. In this paper we explore the phylogenetic structure of Paspalum using sequence data of four non-coding cpDNA fragments from a wide array of species which were combined with morphological data for a subset of diploid taxa. Our results confirmed the general monophyly of Paspalum if P. inaequivalve is excluded and the small genus Thrasyopsis is included. Only one of the four currently recognized subgenera was monophyletic but nested within the remainder of the genus. Some informal morphological groups were found to be polyphyletic. The placement of known allopolyploid groups is generally congruent with previously stated hypotheses although some species with shared genomic formulae formed paraphyletic arrangements. Other species formed a basal grade including mostly umbrophilous or hygrophilous species. It is hypothesized that the genus may have diversified as a consequence of the expansion of C4 grass-dominated grasslands in South America.

Growth form models within the genus Paspalum L. (Poaceae, Paniceae)

Summary Based on a set of morphological features of vegetative axes, some basic patterns (‘models’) are characterized which represent reference points in order to describe the variation of growth forms within the genus Paspalum . This character set comprises duration, differentiation of short and long internodes, number of such internodes, growth direction, presence of adventitious roots, position and relative strength of branches and leaf type. The 19 models ae grouped as follows: A) annual, hapaxanthic species, PU) perennials with unbranched culms, PB) perennials with branched culms, and S) stoloniferous monopodial grasses. Morphological relationships are discussed and some probable evolutionary trends are outlined.

The inflorescences ofPaspalum (Poaceae, Paniceae): TheQuadrifaria group and the evolutionary pathway towards the fully homogenized, truncated common type

The inflorescences ofPaspalum are composed of a main axis and 1 to more than 100 raceme-like lateral branches arranged along it. Each branch bears two rows of homogeneous subunits composed either of one or two consecutive-ordered axes, each one ending with a spikelet. In terms ofTrolls descriptive and typological system, such lateral branches were regarded as long paracladia which bear homogeneous bi-axial short paracladia. The structural pattern of these long paracladia is considered to be a recapitulation of the distal main-axis structure which was lost by evolutionary truncation. — The occurrence of a main florescence (= terminal spikelet) and a short paracladia-bearing zone at the distal portion of the main axis in several species of the genusPaspalum, as it is exemplified by species belonging to the so-called ‘Quadrifaria group’, seems to support the hypothesis that the usual inflorescence ofPaspalum actually derived from a paniculate structure by evolutionary homogenization and truncation processes. This ‘Quadrifaria-type inflorescences’ would be regarded as an intermediate step in the evolutionary pathway. Nevertheless, the phylogenetic implications of this interpretation remains obscure because at the present time there is not any hypothesis about the phylogeny of the genus available.

A Morphology-based Cladistic Analysis of Paspalum sect. Pectinata (Poaceae)

Abstract A cladistic analysis using parsimony was carried out, including all six species of Paspalum sect. Pectinata plus an outgroup composed of six species of Paspalum subg. Ceresia, six additional Paspalum species belonging to different taxonomic groups, and two extrageneric taxa. The analysis was based on 65 morphological characters and was performed using both equal weights and implied weights. In all resulting cladograms, a well supported clade corresponding to Paspalum sect. Pectinata appears, with either P. ceresia or a clade comprising P. ceresia, P. stellatum, and P. eucomum as sister group. Within the Pectinata-clade P. lanciflorum emerges consistently as sister taxon to the remainder of the section, whereas the phylogenetic relationship among them is poorly resolved. The inclusion of Paspalum sect. Pectinata within a weakly supported subgenus Ceresia is confirmed. Incidentally, some doubt is thrown about the validity of the currently accepted circumscription of Paspalum subg. Ceresia, since the inclusion of P. humboldtianum and P. polyphyllum within it is not supported by our data. Communicating Editor: Kathleen Kron

A typological analysis of the inflorescences of the genus Nassauvia (Asteraceae)

Summary The inflorescences of the genus Nassauvia are analyzed following Troll’s comparative-morphological approach. Eight types of synflorescences are recognized, each defined by a particular combination of six developmental processes: truncation, proliferation, shortening of internodes, homogenization of paraclades, suppression of paraclades and shortening of peduncles. Decomposing each inflorescence type into six characters, each referring to a hypothesized developmental process, makes the data suitable for cladistic analysis. The synflorescence patterns described in the present analysis mostly correlate with currently accepted infrageneric classifications; discrepancies are discussed. Inflorescences currently described as „pseudocephalia” are shown to be strongly condensed panicles or stachyoids of heads. Sister group inspection suggests the condensed panicle of heads to be the basal inflorescence condition within the genus Nassauvia.

Cytogenetic analyses in Paspalum L. reveal new diploid species and accessions

Chromosome numbers were counted in 126 new accessions of 50 Paspalum species from Brazil, Argentina, Paraguay and Bolivia. The chromosome numbers 2n=12, 20, 24, 30, 40, 50, 60, 80 were confirmed. Chromosome numbers for P. arenarium (2n=20), P. barretoi (2n=20), P. aff. ceresia (2n=40), P. corcovadense (2n=20), P. crispulum (2n=20), P. flaccidum (2n=40), P. nummularium (2n=20), P. scalare (2n=20), P. vescum (2n=20) and P. rectum (2n=20) and a diploid cytotype of P. malacophyllum are reported for the first time. The predominance of tetraploid accessions (43.6%) was confirmed, but an unusually high number of diploid species (44%) and accessions (35.7%) was found. These results open new perspectives for breeding programs, phylogenetic studies, and for research on apomixis control, since diploids of Paspalum are typically sexual.

Macroevolution of panicoid inflorescences: a history of contingency and order of trait acquisition.

BACKGROUND AND AIMS Inflorescence forms of panicoid grasses (Panicoideae s.s.) are remarkably diverse and they look very labile to human eyes; however, when performing a close inspection one can identify just a small subset of inflorescence types among a huge morphospace of possibilities. Consequently, some evolutionary constraints have restricted, to some extent, the diversification of their inflorescence. Developmental and genetic mechanisms, the photosynthetic type and plant longevity have been postulated as candidate constraints for angiosperms and panicoids in particular; however, it is not clear how these factors operate and which of these have played a key role during the grass inflorescence evolution. To gain insight into this matter the macroevolutionary aspects of panicoid inflorescences are investigated. METHODS The inflorescence aspect (lax versus condensed), homogenization, truncation of the terminal spikelet, plant longevity and photosynthetic type were the traits selected for this study. Maximum likelihood and Bayesian Markov chain Monte Carlo methods were used to test different models of evolution and to evaluate the existence of evolutionary correlation among the traits. Both, models and evolutionary correlation were tested and analysed in a phylogenetic context by plotting the characters on a series of trees. For those cases in which the correlation was confirmed, test of contingency and order of trait acquisition were preformed to explore further the patterns of such co-evolution. KEY RESULTS The data reject the independent model of inflorescence trait evolution and confirmed the existence of evolutionary contingency. The results support the general trend of homogenization being a prerequisite for the loss of the terminal spikelet of the main axis. There was no evidence for temporal order in the gain of homogenization and condensation; consequently, the homogenization and condensation could occur simultaneously. The correlation between inflorescence traits with plant longevity and photosynthetic type is not confirmed. CONCLUSIONS The findings indicate that the lability of the panicoid inflorescence is apparent, not real. The results indicate that the history of the panicoids inflorescence is a combination of inflorescence trait contingency and order of character acquisition. These indicate that developmental and genetic mechanisms may be important constraints that have limited the diversification of the inflorescence form in panicoid grasses.

Centothecoid grasses and the evolution of panicoid spikelets

Abstract.An evolutionary pathway leading to acrotonous, 2-flowered spikelets of Panicoideae has been suggested elsewhere, which involves apical reduction of many-flowered mesotonic spikelets. Current phylogenies of the grass family show a sister relationship between Panicoideae and Centothecoideae. A survey of spikelet structures occurring among centothecoid grasses shows that some representatives of this group have intermediate morphologies which are consistent with that hypothesis. Chasmanthium and Bromuniola have many-flowered spikelets with a barren proximal floret, whereas Thysanolaena, Gouldochloa and Gynerium represent a series of apical reductions leading to 2-flowered spikelets. Moreover, many-flowered spikelets with 1-3 proximal male flowers followed by several female-fertile ones occur in Puelioideae, one of the early-diverging clades of the Poaceae. This fact suggests that some “panicoid” characters may have evolved long before the radiation of the Panicoideae took place.

Growth forms, branching patterns, and inflorescence structure in Digitaria sect. Trichachne (Poaceae, Paniceae).

Summary A survey of branching patterns occurring within Digitaria sect. Trichachne is presented. The general growth form type can be characterized as ‘lax tuft’. The proximal portion of the tillers is typically plagiotropous and bears short internodia. It is usually thickened and corm-like. Nevertheless, in D. swalleniana and D. catamarcensis the internodia are elongated and form a well-developed rhizome. In all cases this region of the culm bears cataphylls and behaves as an innovation zone. The culms grow upright to form synflorescences composed of a main flowering unit which can be accompanied by proximal enrichment shoots. As usual among grasses, the synflorescences are polytelic, and comprise a distal portion bearing short paraclades composed each of two spikelets, and a proximal portion bearing long paraclades, each reproducing the structure of the distal portion. All axes bear terminal spikelets. The number of long paraclades shows a wide range of variation within the section, ranging from a 1 or 2 paraclades in D. tenuis and D. brownii to about 50 in D. insularis and D. laxa . Second order long paraclades only were observed in D. laxa . The short paraclades occur always in two rows along inflorescence axes, but the arrangement of long paraclades is usually polystichous. Some controversial issues related to branching patterns are discussed on the basis of SEM-observations.