Genome size variation and species relationships in the genus Hydrangea

Genome size and base composition in 16 species and subspecies of the Hydrangea, a woody ornamental genus of Hydrangeaceae, were evaluated by flow cytometry in relation to their chromosome number. This is the first such study concerning the genome size of these species together with a karyotype study of the most important species, Hydrangea macrophylla subsp. macrophylla (Hortensia), from an economical point of view. The 2C DNA content ranged from 1.95 pg in Hydrangea quercifolia to 5.00 pg in Hydrangea involucrata. The base composition ranged from 39.9% GC in Hydrangea aspera subsp. sargentiana to 41.1% in Hydrangea scandens subsp. scandens (significant difference at p < 0.05). The smallest genome sizes were those of the three species originating from North or South America. Most of the species studied presented a chromosome number of 2n = 2x = 36, except for those of the section Aspereae which showed 2n = 30, 34 and 36. A primary karyotype has been made for the first time for H. macrophylla subsp. macrophylla. Phylogenetic relationships between species, the origin of chromosome number and an exploration of the genetic diversity within the genus are discussed. Received: 24 July 2000 / Accepted: 31 October 2000     

Genome size variation and polyploidy in the resurrection plant genus Ramonda: Cytogeography of living fossils

The genus Ramonda includes three preglacial paleoendemic species surviving as the rare resurrection angiosperms of the Northern hemisphere in refugia habitats in the Balkan (Ramonda nathaliae and Ramonda serbica) and Iberian Peninsulas (Ramonda myconi). This study focuses on: assessing genome size and base composition, determining chromosome number and ploidy level in several populations, evaluating inter- and intra-specific variations in DNA content and chromosome number as well as looking for the possible hybridization in the sympatric zones of Balkan species. R. nathaliae and R. myconi are diploid species (2n = 2x = 48) while R. serbica is hexaploid (2n = 6x = 144). The mean 2C DNA values ranged from 2.30 pg for R. nathaliae to 2.59 pg for R. myconi compared to 7.91 pg for R. serbica. The base composition for R. nathaliae was 42.1% GC, for R. myconi 39.9% and for R. serbica 41.2%. In one population of R. serbica the DNA content ranged from 2C = 7.65 to 11.82 pg, revealing different ploidy levels among its individuals. In sympatric populations genome size was intermediary (～5 pg) between the diploid and hexaploid classes which indicates the hybridization ability between R. serbica and R. nathaliae. It appears that polyploidization is the major evolutionary mechanism in the genus Ramonda.     

Genome size and karyotype length in some interstitial polychaete species of the genus Ophryotrocha (Dorvilleidae).

Interstitial polychaetes of the genus Ophryotrocha are very small, progenetic, and morphologically very similar. These worms have been widely used in evolutionary biology and sexuality studies. To have a better insight into the karyological evolution of this genus, we measured the total karyotypic length and the 2C nuclear DNA content of the nine best-known species of this genus. No interspecific differences were observed in karyotypic lengths, apart from that of O. gracilis, which was significantly greater than the karyotypic length of five of the nine species. The genome size (i.e., 1C DNA content calculated from 2C DNA content) in eight of the nine species is about 0.4 pg, irrespective of the chromosome number. A group of four gonochoric and morphologically indistinguishable species, with 2n = 6 metacentric chromosomes, appears to be heterogeneous with regard to its DNA content, because one of the species, O. macrovifera, has a genome twice the size of that of the other three species. A hermaphroditic species, O. hartmanni, has a genome three times that size. No correlation has been observed between genome size and body size, egg cell diameter, or time interval from egg fertilization to sexual maturity. The basic genome size of 0.4 pg is among the lowest recorded in invertebrates. Hypotheses about selective pressures that maintain such a low amount of nuclear DNA in this genus are discussed.     

Genome structure of the genus Azospirillum

Azospirillum species are plant-associated diazotrophs of the alpha subclass of Proteobacteria. The genomes of five of the six Azospirillum species were analyzed by pulsed-field gel electrophoresis. All strains possessed several megareplicons, some probably linear, and 16S ribosomal DNA hybridization indicated multiple chromosomes in genomes ranging in size from 4.8 to 9.7 Mbp. The nifHDK operon was identified in the largest replicon.     

Genome size variations within Dasypyrum villosum: correlations with chromosomal traits, environmental factors and plant phenotypic characteristics and behaviour in reproduction

 Feulgen/DNA cytophotometric determinations carried out in the root meristem of seedlings showed that substantial quantitative alterations in the nuclear genome are present between and within 15 natural populations of Dasypyrum villosum in Italy. When the most variant values are considered, there is a 17.6% difference between the mean genome size of the populations, and a 66.2% difference between the genome size of individual plants within a population. A highly significant, positive correlation was found to exist between the genome size of D. villosum plants and the altitude of their stations, and differences in DNA contents between individual plants were greater in populations from mountain sites. Karyological analyses showed all chromosome pairs to differ largely in size between plants with differing DNA contents. A highly significant, positive correlation was found to exist between genome size and both the length of the chromosome complement at metaphase and the length and arm ratio of pair VII. Significant correlations were also found between DNA content and certain phenotypic characteristics of the plants. The mean genome size of the populations was negatively correlated with the mean leaf length and width. In contrast, the genome size of individual plants was positively correlated with the weight of the seed from which they originated and their flowering interval. A large range of genome sizes was found in the half-sib progeny of a plant having a relatively large genome. In contrast, in the half-sib progeny of a plant having a small genome, the genome sizes of the individual plants were less divergent and similar to that of the mother plant. All siblings from crosses between plants with differing genome sizes had similar DNA contents, which were intermediate between those of the parental plants, even if closer to the DNA content of the parent plant having the smaller genome size. Size polymorphism within pairs was never observed in plants obtained from these crosses or in half-sibs whose genome size differed from that of the mother plant. The intraspecific alterations observed in the nuclear genome and their effects on plant development and phenotype are briefly discussed as evolutionary factors which allow D. villosum populations to withstand different environmental conditions as well as the variability of conditions in a given environment. Received: 6 October 1997 / Accepted: 28 October 1997     

Genome size in birds

Nuclear DNA amounts of twenty-three species of birds from seventeen families of seven orders were determined by Feulgen cytophotometry. Genome size is constant in these birds, the ratio between the largest and smallest genome in the sample is 1.3 to 1. The modal diploid DNA amount for birds is about 3.6 picograms, slightly higher than previously reported. The data point towards an evolutionary control of genome size regardless of chromosome number. Birds represent an example of a group in which reduction of genome size is correlated with active speciation.     

Genome size in mammals

Nuclear DNA amounts of fifteen species of placental mammals were determined by Feulgen cytophotometry. Relative values for several widely used species have been ascertained with an error of only a few percent. Absolute values (picograms or numbers of nucleotide pairs) can be determined with an error of about ten percent. The largest known mammalian genome contains about twice the DNA of the smallest one. The modal diploid DNA amount for mammals is slightly above eight picograms.     

Genome size in gymnosperms

The DNA 2C and per chromosome values of 57 species belonging to 22 genera of gymnosperms have been analysed. The overall range is 12-fold with a modal value of about 30.0 pg.Cycadales exhibit a 2-fold difference. AmongConiferales with a 4-fold variation, thePinaceae have higher mean DNA contents as well as a greater range and diversity than other families. Remarkable interspecific differences are found inCycas, Picea, Larix, Pinus, Callitris, Cupressus, andChamaecyparis. Despite this, there is a constancy of basikaryotypes within these genera.Gnetum shows a distinctly low DNA value.     

Genome size and evolution

Examination of data on genome size for prokaryotic cells suggests an evolutionary scheme.     

Genome size stability among five subspecies of Pinus nigra Arnold s.l.

Despite the fact that genome size should be constant at species level, many reports of intraspecific variations exist. Thus, we carried out an analysis to determine the possible existence of nuclear DNA content variation in European black pine (Pinus nigra s.l.), a good model for such a study given its karyological uniformity, morphological polymorphism, broad geographical distribution, ecological plasticity and taxonomic heterogeneity. The panel comprised 20 populations across the natural range of P. nigra from Europe, Northwest Africa and Asia Minor including five subspecies: subsp. nigra, salzmanni, dalmatica, pallasiana and mauretanica. Mean 1C DNA content of the species was 23.62 pg (±0.209) assessed by flow cytometry. This converts to 23.1 G base pairs. The coefficients of variation within and between populations did not exceed 2.6%. Although we had already reported the existence of significant differences for three Black pine populations in our previous work on five Pinus spp. [Bogunic, F., Muratovic, E., Brown, S.C., Siljak-Yakovlev, S., 2003. Genome size of five Pinus from Balkan region. Plant Cell Rep. 22, 59–63], intraspecific variation was not confirmed in the present study dealing with many more populations. Subspecific divisions of Black pine were characterised with following mean values: subsp. pallasiana—23.80 pg, dalmatica—23.79 pg, nigra—23.65 pg, salzmanni—23.55 pg, and mauretanica—23.24 pg. A positive relationship between genome size and longitude was observed (r = 0.44, p < 0.05). We conclude that the diversification of populations of P. nigra has occurred without significant genome size changes throughout its wide geographical range from ecologically contrasting habitats. A clinal mode of genome size variation is present, in line with hypothesis of P. nigra spreading from south-western Asia towards European habitats.     

Genome comparisons in the genus Dipodascus de Lagerheim

Phenotypic characteristics of physiology and morphology of 71 strains belonging to the genus Dipodascus de Lagerheim were examined. The GC contents of genomic DNAs of 46 strains were calculated from the thermal denaturation curves using the spectrophotometric method. The first derivatives of the melting curves revealed that the DNAs of these strains are heterogeneous; four categories could be recognized. However, DNA similarity values calculated by using DNA-DNA reassociation kinetics showed that each category could be subdivided further. Two categories were separated into four subgroups each; the other two yielded five subgroups each. Strains belonging to the same subgroup exhibited high levels of DNA similarity ranging from 82 to 100%. The 18 subgroups represented 13 currently accepted Dipodascus species and five anamorphic Geotrichum species, four representing novel taxa. A phenotypic key to distinguish the taxa of Dipodascus, Galactomyces and Geotrichum is presented.     

Genome size and plastid trnK-matK markers give new insights into the evolutionary history of the genus Lavandula L.

The genus Lavandula L. consists of 39 species distributed from the North Atlantic Islands, across the Mediterranean Basin to India. We analysed 36 taxa of the genus Lavandula representing two of the three subgenera and six of the eight sections according to the most recent classification (Upson & Andrews 2004). We achieved a phylogenetic reconstruction from partial sequences from plastid trnK and matK genes; the genome size was estimated by flow cytometer measurements. The primary aim was to track phylogenetic patterns through the maternal inherited marker at the sectional level and identify possible genome duplications. The cpDNA tree shows the phylogenetic relationships between subgenus, sections and also elucidates for the first time the relationships between the endemic species of Macaronesia, Morocco and Arabia. The ancestral split between the two subgenera could be explained by dispersal followed by an early vicariance event. The C-value shows genome up-sizing within several phylogenetic clades and geographical areas. An ancestral genome-up sizing is characterized at the node of section Dentatae and Lavandula. The cpDNA tree suggests that the taxa L. angustifolia subsp. pyrenaica (DC.) Guinea and L. stoechas subsp. luiseiri are best treated as a distinct species.     

Genome size variation in Tardigrada

The genome size of tardigrades has been documented to date in seven species of eutardigrades, and it is among the lowest in invertebrates. In this paper the evaluation of genome size is extended to eight other speeies of eutardigrades and to one species of heterolardigradcs, in order to obtain a more comprehensive picture of minimum DNA content evolution in this phylum. The genome size of the heterotardigrade Pseudechiniscus juanitae (0.6 pg), measured using le cells, is found to resemble that of some eutardigrades. Thus, it is not possible to discriminate between the two classes of tardigrades on this basis. Literature and new data on eutardigrades also suggest that families or subfamilies cannot be discriminated on the basis of genome size. Nevertheless, genome size and sperm cell shape are correlated. A lower DNA content always corresponds to a more specialized male gamete. In general, in tardigrades, the low genome size and its low variation could be related to the high specialization of the phylum. Preliminary data on nuclear AT contents suggest that the variations in those values are correlated with genome size variations.     

Genome size and developmental complexity

Haploid genome size (C-value) is correlated positively with cell size, and negatively with cell division rate, in a variety of taxa. Because these associations are causative, genome size has the potential to impact (and in turn, be influenced by) organism-level characters affected by variation in either of these cell-level parameters. One such organismal feature is development. Developmental rate, in particular, has been associated with genome size in numerous plant, vertebrate, and invertebrate groups. However, rate is only one side of the developmental coin; the other important component is complexity. When developmental complexity is held essentially constant, as among many plants, developmental rate is the visibly relevant parameter. In this case, genome size can impose thresholds on developmental lifestyle (and vice versa), as among annual versus perennial plants. When developmental rate is constrained (as during time-limited amphibian metamorphosis), complexity becomes the notable variable. An appreciation for this rate-complexity interaction has so far been lacking, but is essential for an understanding of the relationships between genome size and development. Moreover, such an expanded view may help to explain patterns of variation in taxa as diverse as insects and fish. In each case, a hierarchical approach is necessary which recognizes the complex interaction of evolutionary processes operating at several levels of biological organization.