Phylogenetic relationships of basal monocots and the evolution of genome size.

Abstract

The diversification of monocotyledons into major lines, and the evolution of genome size have been studied in this work. The diversification of major lineages of monocots was examined through the phylogenetic analysis of morphological and molecular data. Data on genome sizes were collected and assessed in the context of these hypotheses of relationships. Parsimony analyses of morphological and rDNA sequence data suggest that there are three major groups within monocots: "Liliiflorae," the Alismatid-Aroid group, and a group of derived monocots, "eumonocots". The morphological data suggest that "Liliiflorae" is basal and paraphyletic, and Dioscoreales is one of the earliest branches. However, molecular data suggest a basal bifurcation into the Alismatiflorae-Araceae and the Liliiflorae-"eumonocot" lineages. According to molecular data, Acorus is linked with Piperales, an anomalous position. In morphological analyses it is in a traditional position in the Alismatid-Aroid clade. Different types of exploratory analyses led to the assessment that support for both hypotheses is relatively weak. The overall conclusion is that studies of the evolutionary history of monocots are preliminary in nature. Much needs to be done, in collecting more information on little known taxa and characters, and in identifying new characters (both molecular and morphological), before conclusive statements can be made about monocot relationships. Genome sizes of paleoherb species were determined using flow-cytometry. The data were optimized under parsimony, on different hypotheses of monocot relationships. Despite disagreement among the different phylogenetic results, several conclusions can be made about the evolution of genome size in this group. There was no major increase in genome size during the evolution of monocots. The ancestral condition in most lineages is to have small genomes. Large genomes (2C > 18 pg) occur in only a few lines. Both substantial increases and decreases are noted within monocots. Phylogenetically based statistical tests showed that bisporic and tetrasporic embryo sacs tend to evolve in clades with large genomes. The mechanistic basis for this correlation is unknown. It is hypothesized that the developmental variation may be connected with variation in cell cycle and size, both of which are correlated with genome size.