Animal Genome Size Research Articles

Genome size estimation and its associations with body length, chromosome number and evolution in teleost fishes

Precise estimation of genome size (GS) is vital for various genomic studies, such as deciding genome sequencing depth, genome assembly, biodiversity documentation, evolution, genetic disorders studies, duplication events etc. Animal Genome Size Database provides GS of over 2050 fish species, which ranges from 0.35 pg in pufferfish (Tetraodon nigroviridis) to 132.83 pg in marbled lungfish (Protopterus aethiopicus). The GS of majority of the fishes inhabiting waters of Indian subcontinent are still missing. In present study, we estimated GS of 51 freshwater teleost (31 commercially important, 7 vulnerable and 13 ornamental species) that ranged from 0.58 pg in banded gourami (Trichogaster fasciata) to 1.92 pg in scribbled goby (Awaous grammepomus). Substantial variation in GS was observed within the same fish orders (0.64–1.45 pg in cypriniformes, 0.70–1.41 pg in siluriformes and 0.58–1.92 pg in perciformes). We examined the relationship between the GS, chromosome number and body length across all the fishes. Body length was found to be associated with GS, whereas no relationship was noticed between the GS and the chromosome number. The analysis using ancestral information revealed haploid chromosome number 25, 27 and 24 for the most recent common ancestor of cypriniformes, siluriformes and perciformes, respectively. The study led to generation of new records on GS of 43 fish species and revalidated records for 8 species. The finding is valuable resource for further research in the areas of fish genomics, molecular ecology and evolutionary conservation genetics.

Does genome size increase with water depth in marine fishes?

A growing body of research suggests that genome size in animals can be affected by ecological factors. Half a century ago, Ebeling etal. proposed that genome size increases with depth in some teleost fish groups and discussed a number of biological mechanisms that may explain this pattern (e.g., passive accumulation, adaptive acclimation). Using phylogenetic comparative approaches, we revisit this hypothesis based on genome size and ecological data from up to 708 marine fish species in combination with a set of large-scale phylogenies, including a newly inferred tree. We also conduct modeling approaches of trait evolution and implement a variety of regression analyses to assess the relationship between genome size and depth. Our reanalysis of Ebeling etal.'s dataset shows a weak association between these variables, but the overall pattern in their data is driven by a single clade. Although new analyses based on our "all-species" dataset resulted in positive correlations, providing some evidence that genome size evolves as a function of depth, only one subclade consistently yielded statistically significant correlations. By contrast, negative correlations are rare and nonsignificant. All in all, we find modest evidence for an increase in genome size along the depth axis in marine fishes. We discuss some mechanistic explanations for the observed trends.

Cytogenetic studies in three octopods, Octopusminor, Amphioctopusfangsiao, and Cistopuschinensis from the coast of China.

To provide markers to identify chromosomes in the genome of octopods, chromosomes of three octopus species were subjected to NOR/C-banding. In addition, we examined their genome size (C value) to submit it to the Animal Genome Size Database. Silver staining revealed that the number of Ag-nucleoli was 2 (Octopusminor (Sasaki, 1920)), 2 (Amphioctopusfangsiao (d’Orbigny, 1839)) and 1 (Cistopuschinensis Zheng et al., 2012), respectively, and the number of Ag-nucleoli visible was the same as that of Ag-NORs on metaphase plates in the same species. In all analyzed metaphases, Ag-NORs were mainly located terminally on the long arms of chromosomes 3 (3rd) of O.minor and on the short arms of chromosomes 4 (4th) of A.fangsiao, whereas only one of the chromosomes 23 (23rd) was found Ag-NORs of C.chinensis. C-bands were localized predominantly in the centromeric regions of chromosomes in the three species, while other conspicuous stable C-bands were observed in terminal regions, including the Ag-NORs. That means these three chromosome pairs (3rd, 4th and 23rd) could be considered species-specific cytogenetic markers. The mean C values of O.minor, A.fangsiao and C.chinensis were 7.81±0.39 pg (0.070 pg per unit length), 8.31±0.18 pg (0.068 pg per unit length) and 5.29±0.10 pg (0.038 pg per unit length), respectively, and results showed that C values of the three species were not proportional to the relative length of the chromosomes. These cytogenetic characteristics will provide more theoretical foundation for further researches on chromosome evolution in octopods.

Volume 85A, Number 10, October 2014 Cover Image

On the cover: Arthropods are the most biologically successful animals due to their unsurpassed diversity and adaptability to different Earth environments. For the same reasons, it's hard to obtain and transport fresh or flash frozen samples of arthropod tissues for accurate genome size estimation, leaving a large chunk of this super‐phylum out of the Animal Genome Size Database. A quickly dried leaf is a good source of the intact plant genome for the size estimation, but this approach is not suitable for animal samples. Feulgen densitometry and a good old ethanol preservation appear to be a perfect combination to overcome this issue as shown in two test studies, allowing the addition of 29 species of crustaceans to the database. Read the accompanying article by Jeffery and Gregory in this issue.Cover design by Bärbel Beran [www.beran‐design.de].

Reassessment of genome size in turtle and crocodile based on chromosome measurement by flow karyotyping: close similarity to chicken

The genome size in turtles and crocodiles is thought to be much larger than the 1.2 Gb of the chicken (Gallus gallus domesticus, GGA), according to the animal genome size database. However, GGA macrochromosomes show extensive homology in the karyotypes of the red eared slider (Trachemys scripta elegans, TSC) and the Nile crocodile (Crocodylus niloticus, CNI), and bird and reptile genomes have been highly conserved during evolution. In this study, size and GC content of all chromosomes are measured from the flow karyotypes of GGA, TSC and CNI. Genome sizes estimated from the total chromosome size demonstrate that TSC and CNI are 1.21 Gb and 1.29 Gb, respectively. This refines previous overestimations and reveals similar genome sizes in chicken, turtle and crocodile. Analysis of chromosome GC content in each of these three species shows a higher GC content in smaller chromosomes than in larger chromosomes. This contrasts with mammals and squamates in which GC content does not correlate with chromosome size. These data suggest that a common ancestor of birds, turtles and crocodiles had a small genome size and a chromosomal size-dependent GC bias, distinct from the squamate lineage.

A guided tour of large genome size in animals: what we know and where we are heading

The study of genome size diversity is an ever-expanding field that is highly relevant in today's world of rapid and efficient DNA sequencing. Animal genome sizes range from 0.02 to 132.83 pg but the majority of animal genomes are small, with the most of these genome sizes being less than 5 pg. Animals with large genomes (> 10 pg) are scattered within some invertebrates, including the Platyhelminthes, crustaceans, and orthopterans, and also the vertebrates including the Actinopterygii, Chondrichthyes, and some amphibians. In this paper, we explore the connections between organismal phenotype, physiology, and ecology to genome size. We also discuss some of the molecular mechanisms of genome shrinkage and expansion obtained through comparative studies of species with full genome sequences and how this may apply to species with large genomes. As most animal species sequenced to date have been in the small range for genome size (especially invertebrates) due to sequencing costs and to difficulties associated with large genome assemblies, an understanding of the structural composition of large genomes is still lacking. Studies using next-generation sequencing are being attempted for the first time in animals with larger genomes. Such analyses using low genome coverage are providing a glimpse of the composition of repetitive elements in animals with more complex genomes. These future studies will allow a better understanding of factors leading to genomic obesity in animals.

The quest for suitable reference standards in genome size research

in the molecular genomics era, as the flood gates opento deliver a deluge of DNA sequence data, knowledge of thetotal amount of DNA that comprises an organism’s genome(5 genome size) remains vital for many diverse fields of biol-ogy. Indeed, it may be considered to be a key biodiversitycharacter with both practical and biological consequences forthe organism. Knowledge of genome size is important, forexample, in ongoing and planned sequencing programs toassess costs and to know how much DNA to sequence, or fordetermining how many large insert clones (e.g., bacterial arti-ficial chromosomes [BACs] or fosmids) are needed for con-structing genomic libraries. In addition, various genetic finger-printing tools (e.g., microsatellites and AFLPs) have beenshown to be sensitive to genome size so knowledge of thischaracter is important before embarking on such studies.From a biological perspective variation in genome size hasbeen shown to have diverse yet predictable consequences foran organism, influencing, for example, how it will respondto changes in carbon dioxide, rising temperatures, andpollution (1).While people have been estimating genome sizes in plantsand animals for over 50 years, the last decade has seen a hugegrowth in the number of estimates published, not only to pro-vide data for molecular studies but also for large scale compar-ative analyses seeking to understand the biological and evolu-tionary significance of the 40,000-fold range of genome sizesencountered across eukaryotes (2). Indeed, data are now avail-able for over 10,000 species, accessible through the internetin three databases (Animal Genome Size Database—www.genomesize.com; Fungal Genome Size Database—www.zbi.ee/fungal-genomesize; and the Plant DNA C-values Database—http://data.kew.org/cvalues).

Patterns of genome size diversity in the ray-finned fishes

The ray-finned fishes make up about half of all vertebrate diversity and are by far the best represented group in the Animal Genome Size Database. However, they have traditionally been the least well investigated among vertebrates in terms of patterns and consequences of genome size diversity. This article synthesizes and expands upon existing information about genome size diversity in ray-finned fishes. Specifically, compiled data from the Animal Genome Size Database and FishBase are used to examine the potential patterns of interspecific genome size variability according to ecology, environment, morphology, growth, physiology, reproduction, longevity, and taxonomic diversity. Polyploidy and haploid genome sizes are considered separately, revealing differences in their respective consequences. This represents the most comprehensive summary of fish genome size diversity presented to date, and highlights areas of particular interest to investigate as more data become available.

Eukaryotic genome size databases.

Three independent databases of eukaryotic genome size information have been launched or re-released in updated form since 2005: the Plant DNA C-values Database (), the Animal Genome Size Database () and the Fungal Genome Size Database (). In total, these databases provide freely accessible genome size data for >10 000 species of eukaryotes assembled from more than 50 years' worth of literature. Such data are of significant importance to the genomics and broader scientific community as fundamental features of genome structure, for genomics-based comparative biodiversity studies, and as direct estimators of the cost of complete sequencing programs.

Alternative Splice Variants Encoding Unstable Protein Domains Exist in the Human Brain

Alternative splicing has been recognized as a major mechanism by which protein diversity is increased without significantly increasing genome size in animals and has crucial medical implications, as many alternative splice variants are known to cause diseases. Despite the importance of knowing what structural changes alternative splicing introduces to the encoded proteins for the consideration of its significance, the problem has not been adequately explored. Therefore, we systematically examined the structures of the proteins encoded by the alternative splice variants in the HUGE protein database derived from long (>4 kb) human brain cDNAs. Limiting our analyses to reliable alternative splice junctions, we found alternative splice junctions to have a slight tendency to avoid the interior of SCOP domains and a strong statistically significant tendency to coincide with SCOP domain boundaries. These findings reflect the occurrence of some alternative splicing events that utilize protein structural units as a cassette. However, 50 cases were identified in which SCOP domains are disrupted in the middle by alternative splicing. In six of the cases, insertions are introduced at the molecular surface, presumably affecting protein functions, while in 11 of the cases alternatively spliced variants were found to encode pairs of stable and unstable proteins. The mRNAs encoding such unstable proteins are much less abundant than those encoding stable proteins and tend not to have corresponding mRNAs in non-primate species. We propose that most unstable proteins encoded by alternative splice variants lack normal functions and are an evolutionary dead-end.

Mutational Equilibrium Model of Genome Size Evolution

The paper describes a mutational equilibrium model of genome size evolution. This model is different from both adaptive and junk DNA models of genome size evolution in that it does not assume that genome size is maintained either by positive or stabilizing selection for the optimum genome size (as in adaptive theories) or by purifying selection against too much junk DNA (as in junk DNA theories). Instead the genome size is suggested to evolve until the loss of DNA through more frequent small deletions is equal to the rate of DNA gain through more frequent long insertions. The empirical basis for this theory is the finding of a strong correlation and of a clear power-function relationship between the rate of mutational DNA loss (per bp) through small deletions and genome size in animals. Genome size scales as a negative 1.3 power function of the deletion rate per nucleotide. Such a relationship is not predicted by either adaptive or junk DNA theories. However, if genome size is maintained at equilibrium by the balance of mutational forces, this empirilical relationship can be readily accommodated. Within this framework, this finding would imply that the rate of DNA gain through large insertions scales up a quarter-power function of genome size. On this view, as genome size grows, the rate of growth through large insertions is increasing as a quarter power function of genome size and the rate of DNA loss through small deletions increases linearly, until eventually, at the stable equilibrium genome size value, rates of growth and loss equal each other. The current data also suggest that the long-term variation is genome size in animals is brought about to a significant extent by changes in the intrinsic rates of DNA loss through small deletions. Both the origin of mutational biases and the adaptive consequences of such a mode of evolution of genome size are discussed.