Variation in DNA C value in natural populations of Dactylis glomerata L

Genome size variation and evolution during invasive range expansion in an introduced plant

Plants demonstrate exceptional variation in genome size across species, and their genome sizes can also vary dramatically across individuals and populations within species. This aspect of genetic variation can have consequences for traits and fitness, but few studies attributed genome size differentiation to ecological and evolutionary processes. Biological invasions present particularly useful natural laboratories to infer selective agents that might drive genome size shifts across environments and population histories. Here, we test hypotheses for the evolutionary causes of genome size variation across 14 invading populations of yellow starthistle, Centaurea solstitialis, in California, United States. We use a survey of genome sizes and trait variation to ask: (1) Is variation in genome size associated with developmental trait variation? (2) Are genome sizes smaller toward the leading edge of the expansion, consistent with selection for "colonizer" traits? Or alternatively, does genome size increase toward the leading edge of the expansion, consistent with predicted consequences of founder effects and drift? (3) Finally, are genome sizes smaller at higher elevations, consistent with selection for shorter development times? We found that 2C DNA content varied 1.21-fold among all samples, and was associated with flowering time variation, such that plants with larger genomes reproduced later, with lower lifetime capitula production. Genome sizes increased toward the leading edge of the invasion, but tended to decrease at higher elevations, consistent with genetic drift during range expansion but potentially strong selection for smaller genomes and faster development time at higher elevations. These results demonstrate how genome size variation can contribute to traits directly tied to reproductive success, and how selection and drift can shape that variation. We highlight the influence of genome size on dynamics underlying a rapid range expansion in a highly problematic invasive plant.

On the Origin of Tetraploid Vernal Grasses (Anthoxanthum) in Europe

Polyploidy has played a crucial role in the evolution of many plant taxa, namely in higher latitudinal zones. Surprisingly, after several decades of an intensive research on polyploids, there are still common polyploid species whose evolutionary history is virtually unknown. Here, we addressed the origin of sweet vernal grass (Anthoxanthum odoratum) using flow cytometry, DNA sequencing, and in situ hybridization-based cytogenetic techniques. An allotetraploid and polytopic origin of the species has been verified. The chromosome study reveals an extensive variation between the European populations. In contrast, an autopolyploid origin of the rarer tetraploid vernal grass species, A. alpinum, has been corroborated. Diploid A. alpinum played an essential role in the polyploidization of both European tetraploids studied.

Population Genomic Approaches for Weed Science

Genomic approaches are opening avenues for understanding all aspects of biological life, especially as they begin to be applied to multiple individuals and populations. However, these approaches typically depend on the availability of a sequenced genome for the species of interest. While the number of genomes being sequenced is exploding, one group that has lagged behind are weeds. Although the power of genomic approaches for weed science has been recognized, what is needed to implement these approaches is unfamiliar to many weed scientists. In this review we attempt to address this problem by providing a primer on genome sequencing and provide examples of how genomics can help answer key questions in weed science such as: (1) Where do agricultural weeds come from; (2) what genes underlie herbicide resistance; and, more speculatively, (3) can we alter weed populations to make them easier to control? This review is intended as an introduction to orient weed scientists who are thinking about initiating genome sequencing projects to better understand weed populations, to highlight recent publications that illustrate the potential for these methods, and to provide direction to key tools and literature that will facilitate the development and execution of weed genomic projects.

Genome Size Unaffected by Variation in Morphological Traits, Temperature, and Precipitation in Turnip

Genome size (GS) was proposed as proxy for gross phenotypic and environmental changes in plants. GS organismal complexity is an enigma in evolutionary biology. While studies pertaining to intraspecific GS variation are abundant, literatures reporting the adaptive significance of GS are largelymissing. During food shortage, Brassica rapa var. rapa (turnip) is used as food and fodder for sustaining the livelihood of residents in the Qinghai Tibetan Plateau (QTP), which is also known as “the roof of the world”. Thus, climatic extremities make this region a natural environment to test adaptive significance of GS variation in turnip landraces. Therefore, from the QTP and its adjacent regions (the Hengduanshan and the Himalayas), we investigated adaptive evolution of GS in turnip landraces. Tuber diameter of turnip landraces was found to be significantly correlated with most of the environmental factors. GS was also shown not to be associated with morphological traits, temperature, and precipitation. Moreover, principal component analyses based on the whole dataset trisected the landraces into three distinct populations based on landrace usage—Hengduanshan, QTP, and the Himalayas. Nonetheless, our cumulative dataset showed evidence of adaptation of turnip landrace to different environments throughnonassociated genomic and phenomic plasticity.

Evolutionary and Taxonomic Implications of Variation in Nuclear Genome Size: Lesson from the Grass Genus Anthoxanthum (Poaceae)

The genus Anthoxanthum (sweet vernal grass, Poaceae) represents a taxonomically intricate polyploid complex with large phenotypic variation and its evolutionary relationships still poorly resolved. In order to get insight into the geographic distribution of ploidy levels and assess the taxonomic value of genome size data, we determined C- and Cx-values in 628 plants representing all currently recognized European species collected from 197 populations in 29 European countries. The flow cytometric estimates were supplemented by conventional chromosome counts. In addition to diploids, we found two low (rare 3x and common 4x) and one high (~16x-18x) polyploid levels. Mean holoploid genome sizes ranged from 5.52 pg in diploid A. alpinum to 44.75 pg in highly polyploid A. amarum, while the size of monoploid genomes ranged from 2.75 pg in tetraploid A. alpinum to 9.19 pg in diploid A. gracile. In contrast to Central and Northern Europe, which harboured only limited cytological variation, a much more complex pattern of genome sizes was revealed in the Mediterranean, particularly in Corsica. Eight taxonomic groups that partly corresponded to traditionally recognized species were delimited based on genome size values and phenotypic variation. Whereas our data supported the merger of A. aristatum and A. ovatum, eastern Mediterranean populations traditionally referred to as diploid A. odoratum were shown to be cytologically distinct, and may represent a new taxon. Autopolyploid origin was suggested for 4x A. alpinum. In contrast, 4x A. odoratum seems to be an allopolyploid, based on the amounts of nuclear DNA. Intraspecific variation in genome size was observed in all recognized species, the most striking example being the A. aristatum/ovatum complex. Altogether, our study showed that genome size can be a useful taxonomic marker in Anthoxathum to not only guide taxonomic decisions but also help resolve evolutionary relationships in this challenging grass genus.

Ploidy distribution and DNA content variations of Lonicera caerulea (caprifoliaceae) in Japan

Ploidy level and geographical distribution were investigated in Japanese Lonicera caerulea L. Flow cytometric analysis revealed the presence of DNA diploid and DNA tetraploid plants in Japan. Chromosome observation confirmed that diploid and tetraploid plants showed 2n = 2x = 8 and 2n =4x = 36, respectively. The DNA diploid populations were found only in lowland mires, Betsukai, Bekanbeushi, Kushiro and Kiritappu located in eastern Hokkaido. On the other hand, DNA tetraploid populations were distributed in a wide area of Hokkaido, and mainland of Japan. The habitats of DNA tetraploid plants were lowland to alpine region. The DNA content measurement with flow cytometry revealed significant differences in the relative DNA contents among DNA tetraploid populations. The relative DNA content within DNA tetraploid populations varied 1.157-fold at maximum, and might correlate with altitude indicating that DNA contents were smaller as altitude increases. The wide area of distribution in various environments of DNA tetraploid plants suggested the adaptability of the tetraploid plants. Although diploid and tetraploid populations were found, no triploid was detected, indicating crossing difficulty between diploid and tetraploid as confirmed by crossing experiment.

Genome size variation in diploid and tetraploid wild wheats

BACKGROUND AND AIMS

Intra- and interspecific variations of C-values and the relationship between habitat factors and genome size were studied in natural populations of diploid and tetraploid wild wheats.

METHODOLOGY

The 1C nuclear DNA content of 376 individual plants representing 41 populations of diploid and tetraploid wild wheats was determined by flow cytometry (FCM) and correlated with geographical and bioclimate variables.

PRINCIPAL RESULTS

Based on analysis of variance, significant differences between diploid and tetraploid Triticum species were found. Differences among populations of T. boeoticum and T. dicoccoides were also statistically significant and argue for isolation between populations, except for T. araraticum. However, the variation among individuals of the same population was not statistically significant. Maximum genome size differences among populations for T. boeoticum (0.143 pg; 2.32 %), T. dicoccoides (0.314 pg; 2.49 %) and T. araraticum (0.116 pg; 0.98 %) argue for genome constancy in these species. There was no significant correlation between intra-population variance and geographical and bioclimate variables for T. boeoticum and T. dicoccoides. In contrast to the limited genome size variation at the intraspecific level, the interspecific variation was large: ∼0.5 pg/1C (8 %) at the diploid level (T. boeoticum vs. T. urartu) and ∼1 pg/1C (9.7 %) at the tetraploid level (T. dicoccoides vs. T. araraticum).

CONCLUSIONS

Low intraspecific genome size variation occurs in diploid and tetraploid wild wheats, and this limited variation is not correlated with geographical and climate variables. However, interspecific variation is significant at the diploid and tetraploid level. It can be concluded that the genome size of wild self-fertilizing Triticum species is generally stable, despite the presence of many potentially active retroelements. In natural habitats, it is very difficult to distinguish wild wheats from each other. However, all four species can be distinguished easily, quickly and unambiguously by using the FCM technique.

Genome size in Hieracium subgenus Hieracium (Asteraceae) is strongly correlated with major phylogenetic groups

BACKGROUND AND AIMS

Hieracium subgenus Hieracium is one of the taxonomically most intricate groups of vascular plants, due to polyploidy and a diversity of breeeding systems (sexuality vs. apomixis). The aim of the present study was to analyse nuclear genome size in a phylogenetic framework and to assess relationships between genome size and ploidy, breeding system and selected ecogeographic features.

METHODS

Holoploid and monoploid genome sizes (C- and Cx-values) of 215 cultivated plants from 89 field populations of 42 so-called 'basic' Hieracium species were determined using propidium iodide flow cytometry. Chromosome counts were available for all analysed plants, and all plants were tested experimentally for their mode of reproduction (sexuality vs. apomixis). For constructing molecular phylogenetic trees, the external transcribed spacer region of nuclear ribosomal DNA was used.

KEY RESULTS

The mean 2C values differed up to 2.37-fold among different species (from 7.03 pg in diploid to 16.67 in tetraploid accessions). The 1Cx values varied 1.22-fold (between 3.51 and 4.34 pg). Variation in 1Cx values between conspecific (species in a broad sense) accessions ranged from 0.24% to 7.2%. Little variation (not exceeding the approximate measurement inaccurracy threshold of 3.5%) was found in 33 species, whereas variation higher than 3.5% was detected in seven species. Most of the latter may have a polytopic origin. Mean 1Cx values of the three cytotypes (2n, 3n and 4n) differed significantly (average of 3.93 pg in diploids, 3.82 pg in triploids and 3.78 pg in tetraploids) indicating downsizing of genomes in polyploids. The pattern of genome size variation correlated well with two major phylogenetic clades which were composed of species with western or eastern European origin. The monoploid genome size in the 'western' species was significantly lower than in the 'eastern' ones. Correlation of genome size with latitude, altitude and selected ecological characters (light and temperature) was not significant. A longitudinal component was only apparent for the whole data set, but absent within the major lineages.

CONCLUSIONS

Phylogeny was the most important factor explaining the pattern of genome size variation in Hieracium sensu stricto, species of western European origin having significantly lower genome size in comparison with those of eastern European origin. Any correlation with ecogeographic variables, including longitude, was outweighed by the divergence of the genus into two major phylogenetic lineages.

Genome size microscale divergence of Cyclamen persicum in Evolution Canyon, Israel

Using DAPI flow cytometry, we examined genome size divergence of the Persian violet, Cyclamen persicum (Primulaceae) (2n=48) on close opposite slopes of Evolution Canyon (EC), Mt. Carmel, Israel. The range of genome size variation detected among measured cyclamens was 6.41% in relation to the smallest measured DNA content. Our data on C. persicum at EC showed that local variability in the 2C-value exists. Significantly less DNA was recorded in plants growing in one station of the African savannah-like south-facing slope (AS) but not in the remaining two stations of the same slope. We were not able to reject the null hypothesis that there are no significant interslope differences in the genome size between the temperate European garrigue-like north-facing slope (ES) and the drier AS. In spite of the nonsignificant interslope trend for the higher genome size in C. persicum, the data-fusion (meta-analysis) test using correlations between C-values in C. persicum, and earlier studied carob tree (Ceratonia siliqua), trifoil (Lotus peregrinus) and a beetle (Oryzaephilus surinamensis) and their distribution along the aridity gradient indicates a positive relationship between drought and genome size at the microsite.

Variation of DNA amount in 47 populations of the subtribe Artemisiinae and related taxa (Asteraceae, Anthemideae): karyological, ecological, and systematic implications

Genome size has been estimated by flow cytometry in 47 populations of 40 species of the tribe Anthemideae (Asteraceae), mainly from Artemisia and other genera of the subtribe Artemisiinae and related taxa. A range of 2C values from 3.54 to 21.22 pg was found. DNA amount per basic chromosome set ranged from 1.77 to 7.70 pg. First genome size estimates are provided for one subtribe, 10 genera, 32 species, and two subspecies. Nuclear DNA amount correlated well with some karyological, physiological and environmental characters, and has been demonstrated as a useful tool in the interpretation of evolutionary relationships within Artemisia and its close relatives.

Evolution of genome size in pines (Pinus) and its life-history correlates: supertree analyses

Genome size has been suggested to be a fundamental biological attribute in determining life-history traits in many groups of organisms. We examined the relationships between pine genome sizes and pine phylogeny, environmental factors (latitude, elevation, annual rainfall), and biological traits (latitudinal and elevational ranges, seed mass, minimum generation time, interval between large seed crops, seed dispersal mode, relative growth rate, measures of potential and actual invasiveness, and level of rarity). Genome sizes were determined for 60 pine taxa and then combined with published values to make a dataset encompassing 85 species, or 70% of species in the genus. Supertrees were constructed using 20 published source phylogenies. Ancestral genome size was estimated as 32 pg. Genome size has apparently remained stable or increased over evolutionary time in subgenus Strobus, while it has decreased in most subsections in subgenus Pinus. We analyzed relationships between genome size and life-history variables using cross-species correlations and phylogenetically independent contrasts derived from supertree constructions. The generally assumed positive relation between genome size and minimum generation time could not be confirmed in phylogenetically controlled analyses. We found that the strongest correlation was between genome size and seed mass. Because the growth quantities specific leaf area and leaf area ratio (and to a lesser extent relative growth rate) are strongly negatively related to seed mass, they were also negatively correlated with genome size. Northern latitudinal limit was negatively correlated with genome size. Invasiveness, particularly of wind-dispersed species, was negatively associated with both genome size and seed mass. Seed mass and its relationships with seed number, dispersal mode, and growth rate contribute greatly to the differences in life-history strategies of pines. Many life-history patterns are therefore indirectly, but consistently, associated with genome size.