Global climate has fluctuated greatly during the past three million years, leading to the recent major ice ages. An inescapable consequence for most living organisms is great changes in their distribution, which are expressed differently in boreal, temperate and tropical zones. Such range changes can be expected to have genetic consequences, and the advent of DNA technology provides most suitable markers to examine these. Several good data sets are now available, which provide tests of expectations, insights into species colonization and unexpected genetic subdivision and mixture of species. The genetic structure of human populations may be viewed in the same context. The present genetic structure of populations, species and communities has been mainly formed by Quaternary ice ages, and genetic, fossil and physical data combined can greatly help our understanding of how organisms were so affected.

http://webpages.icav.up.pt/PTDC/BIA-BEC/105083/2008/hewitt2000.pdf

The genetic legacy of the Quaternary ice ages

An appreciation of the scale and frequency of climatic oscillations in the past few million years is modifying our views on how evolution proceeds. Such major events caused extinction and repeated changes in the ranges of those taxa that survived. Their spatial effects depend on latitude and topography, with extensive extinction and recolonization in higher latitudes and altitudinal shifts and complex refugia nearer the tropics. The associated population dynamics varied with life history and geography, and the present genetic constitution of the populations and species carry attenuated signals of these past dynamics. Phylogeographic studies with DNA have burgeoned recently and studies are reviewed from the arctic, temperate and tropical regions, seeking commonalities of cause in the resulting genetic patterns. Arctic species show distinct shallow genetic clades with common geographical boundaries. Thus Beringia is distinct phylogeographically, but its role as a refugial source is complex. Arctic taxa do not show the common genetic pattern of southern richness and northern purity in north-temperate species. Temperate refugial regions in Europe and North America show relatively deep DNA divergence for many taxa, indicating their presence over several Ice Ages, and suggesting a mode of speciation by repeated allopatry. DNA evidence indicates temperate species in Europe had different patterns of postglacial colonization across the same area and different ones in previous oscillations, whereas the northwest region of North America was colonized from the north, east and south. Tropical montane regions contain deeply diverged lineages, often in a relatively small geographical area, suggesting their survival there from the Pliocene. Our poor understanding of refugial biodiversity would benefit from further combined fossil and genetic studies.

http://webpages.icav.up.pt/PTDC/BIA-BEC/099915/2008/4.Hewitt%202004%20PhilTransRSocLondB.pdf

Genetic consequences of climatic oscillations in the Quaternary.

The {ITS} region of nuclear ribosomal {DNA}: {A} valuable source of evidence on angiosperm phylogeny

The origins and development of the study of speciation, hybrid zones and phylogeography are outlined using evolutionary iconography. This traces the ideas in this field from Lamarck and Darwin through to the present as represented in diagrams and figures. A ‘tree of trees’ summarizes this growth and current vitality. The new facility to use various DNA sequences from nuclear, mitochondrial and chloroplast genomes to determine genetic variation throughout a species range is examined particularly. There is great genomic subdivision across species distributions, which can be interpreted in the light of the recent demonstrations of severe palaeoclimatic oscillations. Refugia and postglacial colonization routes are proposed for several organisms across Europe. The role of geography in speciation through the Pleistocene is considered. These emerging principles and analyses are applied to data available on a variety of organisms in other regions of the world, such as the Arctic, North America and the Tropics, and including the progress of Homo sapiens through the last ice age. Some suggestions are made for future research directions.

http://webpages.icav.up.pt/PTDC/BIA-BEC/104097/2008/7.pdf

Speciation, hybrid zones and phylogeography - or seeing genes in space and time.

iological invasions threaten species and ecosystems world-wide (1). An estimated 50,000 exotic species have beenintroduced to the United States, of which 5,000 are plant speciesthat have escaped and now exist in natural environments (2).Both the actual number of invaders and the impacts of thesespecies may be underestimated because of the presence ofcryptic invaders, or species that cannot be easily classified asnative or introduced (3). Over the past decade, use of PCR-basedmolecular techniques have revealed repeated occurrences ofsuch invasions in marine ecosystems (4–6), and studies havedemonstrated both genetic and physiological differences be-tween invading and native populations (6). Given that crypticinvaders typically are unrecognized or are mistaken for nativespecies, knowledge of historical trends in geographic distributionand population genetic structure in cases of suspected introduc-tions are of particular interest when trying to reconstruct theinvasionhistoryofaspecies.Insuchcases,museumorherbariumspecimens are an invaluable resource for reconstructing popu-lation history.Common reed,

Cryptic invasion by a non-native genotype of the common reed, Phragmites australis, into

Molecular studies of plants from the Pacific Northwest of North America suggest a recurrent pattern of genetic differentiation and geographic structuring. In each of five angiosperms and one fern species representing diverse life histories, cpDNA data indicate two clades of populations that are geographically structured. A northern group comprises populations from Alaska to central or southern Oregon, whereas populations from central Oregon southward to northern California form a southern group. In several of these species, a few populations having southern genotypes may have survived in glacial refugia further north in the Olympic Peninsula, Queen Charlotte Islands, and Prince of Wales Island. Allozyme data reveal a similar pattern of differentiation in several other plants from the Pacific Northwest. North-south partitioning of genotypes has also been reported for several animal species from this region. On a broader geographic scale, northsouth partitioning of genotypes has also been observed in other plants from western North America having a variety of geographic distributions. Some species also display a reduction of genetic variability in the northern portion of their range compared to the south. The data suggest strongly that past glaciation profoundly influenced the genetic architecture of the flora and fauna of the Pacific Northwest. Two alternative hypotheses are advanced to explain the geographic structuring of genotypes. First, past glaciation may have created discontinuities in the geographic distributions of plant species, with populations surviving in several well-isolated northern and southern refugia. Following glaciation, migration of genetically differentiated, once-isolated populations resulted in the formation of a continuous geographic distribution with a major genetic discontinuity. Alternatively, plants survived and subsequently migrated northward from a southern refugium, and a genotype became fixed in one or a few populations at the leading edge of recolonization. Subsequent long-distance dispersal from this leading edge resulted in a relatively uniform northern genotype that differs from the southern genotype(s). Whatever the underlying mechanism, Pleistocence glaciation may have molded the intraspecific genetic architecture of both plants and animals from the Pacific Northwest in a geographically similar manner. Future studies should seek to obtain a comprehensive phylogeography for regions that includes a diversity of both plants and animals.

Chloroplast DNA intraspecific phylogeography of plants from the Pacific Northwest of North America

DNA sequences from the plastid gene rbcL were used to assess relationships among genera of the conifer families Taxodiaceae and Cupressaceae s. str. Phylogenetic analyses produced four most parsimonious trees that differ only in the positions of three genera (Athrotaxis, Taxodium, and Taiwania) and the degree of resolution of a trichotomy within Cupressaceae s. str. Taxodiaceae and Cupressaceae form a monophyletic group. The major lineages of Taxodiaceae diverged first, and a monophyletic Cupressaceae s. str. are derived from within Taxodiaceae. These results are consistent with recent suggestions that the two families be treated as a single family, Cupressaceae s.l. Sciadopitys, often classified in Taxodiaceae, is not closely related to Cupressaceae s.l. and should be excluded from the family. Sequence evolution of rbcL is extremely slow in this group of long- lived trees. The rate of silent nucleotide substitutions is 2.5 x 10" per site per year, approximately 13 times slower than rates estimated in short-lived monocots.

Phylogenetic Relationships among the Genera of Taxodiaceae and Cupressaceae: Evidence from rbcL Sequences

Flavonols are small (C15) plant-specific molecules that are required for petunia and maize pollen to germinate. They exist in two chemical forms: the aglycone or glycosyl conjugates. Flavonol-deficient pollen is biochemically complemented by flavonol aglycones but not by the glycosylated forms that accumulate in wild type (WT) pollen. Coincident with the biochemical induction of germination, the added flavonol aglycone is rapidly converted to a galactoside and then to a glucosyl galactoside (diglycoside) that is identical to the compound present in WT pollen. A flavonol 3-O-galactosyltransferase (F3GalTase) activity has been identified that controls the formation of glycosylated flavonols in pollen. Importantly, this enzyme also catalyzes the reverse reaction, i.e. the production of the flavonol aglycone from the galactoside and UDP (Fig. 1). F3GalTase/RevGalTase therefore has the potential to control the level of the bioactive flavonol species and as a result, pollen germination.

Darren D. Strenge

Papers

Chloroplast DNA intraspecific phylogeography of plants from the Pacific Northwest of North America

Phylogenetic Relationships among the Genera of Taxodiaceae and Cupressaceae: Evidence from rbcL Sequences

The role of glycosylation in flavonol-induced pollen germination