Molecular systematics of Iridaceae: evidence from four plastid DNA regions.

Iridaceae are one of the largest families of Lilianae and probably also among the best studied of monocotyledons. To further evaluate generic, tribal, and subfamilial relationships we have produced four plastid DNA data sets for 57 genera of Iridaceae plus outgroups: rps4, rbcL (both protein-coding genes), the trnL intron, and the trnL-F intergenic spacer. All four matrices produce similar although not identical trees, and we thus analyzed them in a combined analysis, which produced a highly resolved and well-supported topology, in spite of the fact that the partition homogeneity test indicated strong incongruence. In each of the individual trees, some genera or groups of genera are misplaced relative to morphological cladistic studies, but the combined analysis produced a pattern much more similar to these previous ideas of relationships. In the combined tree, all subfamilies were resolved as monophyletic, except Nivenioideae that formed a grade in which Ixioideae were embedded. Achlorophyllous Geosiris (sometimes referred to Geosiridaceae or Burmanniaceae) fell within the nivenioid grade. Most of the tribes were monophyletic, and Isophysis (Tasmanian) was sister to the rest of the family; Diplarrhena (Australian) fell in a well-supported position as sister to Irideae/Sisyrinchieae/Tigridieae/Mariceae (i.e., Iridoideae); Bobartia of Sisyrinchieae is supported as a member of Irideae. The paraphyly of Nivenioideae is suspicious due to extremely high levels of sequence divergence, and when they were constrained to be monophyletic the resulting trees were only slightly less parsimonious (<1.0%). However, this subfamily also lacks clear morphological synapomorphies and is highly heterogeneous, so it is difficult to develop a strong case on nonmolecular grounds for their monophyly.

Use of the polymerase chain reaction to detect spacer size heterogeneity in plant 5S-rRNA gene clusters and to locate such clusters in wheat (Triticum aestivum L.).

We have used the polymerase chain reaction to analyse variation in the size of individual 5S-ribosomal gene spacer sequences. This reaction can be used to demonstrate inter- and intraspecific variation in spacer size, and combined with DNA sequencing it may thus be a valuable taxonomic tool. Two sets of nested polymerase chain reaction primers were designed to amplify the nontranscribed spacer DNA between repeated 5S-rRNA genes. These "universal" primers were used to generate fragments from the genomic DNA from several unrelated monocotyledonous plants. Ribosomal RNA spacer sequences generated in these experiments could also be used to locate 5S-rRNA gene clusters on specific chromosomes in hexaploid wheat (Triticum aestivum). Three distinct spacer sizes were observed after amplification. These were assigned locations on chromosomes by analysing amplification products of genomic DNA from nullisomic/tetrasomic and ditelosomic wheat stocks. "Large" 508-bp 5S repeats are located on the short arm of chromosome 5B and "short" 416-bp and 425-bp repeat unit variants are located on the short arms of chromosomes 1B and 1D, respectively. No other loci were detected. The spacer fragments were cloned, sequenced, and shown to be homologous to wheat 5S-rRNA spacers previously identified. Spacers of uniform size but with some sequence heterogeneity were shown to be located at each locus.

A mechanism for producing magnetic remanence in meteorites and lunar samples by cosmic-ray exposure.

An irradiation of 3 x 1017 neutrons per square centimeter in a reactor core produced an increase in the coercive force of iron and kamacite of 16 to 21 percent. The alternating-current demagnetization spectrum of saturation isothermal remanence was shifted toward higher coercive forces. Similar neutron fluences produced by cosmic-ray exposure may be capable of converting soft isothermal remanence in meteorites and lunar samples to remanence with a higher coercive force.