Ziwen He

Two Evolutionary Histories in the Genome of Rice: the Roles of Domestication Genes

Genealogical patterns in different genomic regions may be different due to the joint influence of gene flow and selection. The existence of two subspecies of cultivated rice provides a unique opportunity for analyzing these effects during domestication. We chose 66 accessions from the three rice taxa (about 22 each from Oryza sativa indica, O. sativa japonica, and O. rufipogon) for whole-genome sequencing. In the search for the signature of selection, we focus on low diversity regions (LDRs) shared by both cultivars. We found that the genealogical histories of these overlapping LDRs are distinct from the genomic background. While indica and japonica genomes generally appear to be of independent origin, many overlapping LDRs may have originated only once, as a result of selection and subsequent introgression. Interestingly, many such LDRs contain only one candidate gene of rice domestication, and several known domestication genes have indeed been “rediscovered” by this approach. In summary, we identified 13 additional candidate genes of domestication.

Population Genetics in Nonmodel Organisms: II. Natural Selection in Marginal Habitats Revealed by Deep Sequencing on Dual Platforms

Population genetics of species living in marginal habitats could be particularly informative about the genetics of adaptation, but such analyses have not been readily feasible until recently. Sonneratia alba, a mangrove species widely distributed in the Indo-West Pacific, provides a very suitable system for the study of local adaptation. In this study, we analyzed DNA variation by pooling 71 genes from 85-100 individuals for DNA sequencing. For each of the two nearby S. alba populations, we obtained ~2,500 × coverage on the Illumina GA platform and for the Sanya population, an additional 5,400 × coverage on the AB SOLiD platform. For the Sanya sample, although each sequencing method called many putative single nucleotide polymorphisms, the two sets of calls did not overlap, suggesting platform-dependent errors. Conventional sequencing corroborated that each population is monomorphic. The two populations differ by 54 bp of 79,000 sites, but 90% of the variants are found in 10% of the genes. Strong local adaptation and high migration may help to explain the extensive monomorphism shared by the two populations in the presence of a small number of highly differentiated loci.

Estimating DNA polymorphism from next generation sequencing data with high error rate by dual sequencing applications

BackgroundAs the error rate is high and the distribution of errors across sites is non-uniform in next generation sequencing (NGS) data, it has been a challenge to estimate DNA polymorphism (θ) accurately from NGS data.ResultsBy computer simulations, we compare the two methods of data acquisition - sequencing each diploid individual separately and sequencing the pooled sample. Under the current NGS error rate, sequencing each individual separately offers little advantage unless the coverage per individual is high (>20X). We hence propose a new method for estimating θ from pooled samples that have been subjected to two separate rounds of DNA sequencing. Since errors from the two sequencing applications are usually non-overlapping, it is possible to separate low frequency polymorphisms from sequencing errors. Simulation results show that the dual applications method is reliable even when the error rate is high and θ is low.ConclusionsIn studies of natural populations where the sequencing coverage is usually modest (~2X per individual), the dual applications method on pooled samples should be a reasonable choice.

Can genomic data alone tell us whether speciation happened with gene flow

The allopatric model, which requires a period of geographical isolation for speciation to complete, has been the standard model in the modern era. Recently, “speciation with gene flow” has been widely discussed in relation to the model of “strict allopatry” and the level of DNA divergence across genomic regions. We wish to caution that genomic data by themselves may only permit the rejection of the simplest form of allopatry. Even a slightly more complex and realistic model that starts with subdivided populations would be impossible to reject by the genomic data alone. To resolve this central issue of speciation, other forms of observations such as the sequencing of reproductive isolation genes or the identification of geographical barrier(s) will be necessary.

De Novo Assembly of Coding Sequences of the Mangrove Palm (Nypa fruticans) Using RNA-Seq and Discovery of Whole-Genome Duplications in the Ancestor of Palms

Nypa fruticans (Arecaceae) is the only monocot species of true mangroves. This species represents the earliest mangrove fossil recorded. How N. fruticans adapts to the harsh and unstable intertidal zone is an interesting question. However, the 60 gene segments deposited in NCBI are insufficient for solving this question. In this study, we sequenced, assembled and annotated the transcriptome of N. fruticans using next-generation sequencing technology. A total of 19,918,800 clean paired-end reads were de novo assembled into 45,368 unigenes with a N50 length of 1,096 bp. A total of 41.35% unigenes were functionally annotated using Blast2GO. Many genes annotated to “response to stress” and 15 putative positively selected genes were identified. Simple sequence repeats were identified and compared with other palms. The divergence time between N. fruticans and other palms was estimated at 75 million years ago using the genomic data, which is consistent with the fossil record. After calculating the synonymous substitution rate between paralogs, we found that two whole-genome duplication events were shared by N. fruticans and other palms. These duplication events provided a large amount of raw material for the more than 2,000 later speciation events in Arecaceae. This study provides a high quality resource for further functional and evolutionary studies of N. fruticans and palms in general.

Pronounced genetic differentiation and recent secondary contact in the mangrove tree Lumnitzera racemosa revealed by population genomic analyses.

Systematically investigating the impacts of Pleistocene sea-level fluctuations on mangrove plants may provide a better understanding of their demographic history and useful information for their conservation. Therefore, we conducted population genomic analyses of 88 nuclear genes to explore the population dynamics of a mangrove tree Lumnitzera racemosa across the Indo-West Pacific region. Our results revealed pronounced genetic differentiation in this species between the populations from the Indian Ocean and the Pacific Ocean, which may be attributable to the long-term isolation between the western and eastern coasts of the Malay Peninsula during sea-level drops in the Pleistocene glacial periods. The mixing of haplotypes from the two highly divergent groups was identified in a Cambodian population at almost all 88 nuclear genes, suggesting genetic admixture of the two lineages at the boundary region. Similar genetic admixture was also found in other populations from Southeast Asia based on the Bayesian clustering analysis of six nuclear genes, which suggests extensive and recent secondary contact of the two divergent lineages in Southeast Asia. Computer simulations indicated substantial migration from the Indian Ocean towards the South China Sea, which likely results in the genetic admixture in Southeast Asia.

Development and characterization of microsatellite loci for smooth cordgrass, Spartina alterniflora (Poaceae).

Premise of the study: Spartina alterniflora is one of the nine most notoriously invasive plants in China. Microsatellite markers were developed for this species to investigate its invasiveness and genetic diversity. Methods and Results: Fifteen polymorphic and seven monomorphic simple sequence repeat (SSR) markers derived from expressed sequence tags (ESTs) were identified and screened in 60 samples of S. alterniflora. The number of alleles per polymorphic locus ranged from two to eight, with an average of 3.8 alleles per polymorphic locus. The expected heterozygosity and observed heterozygosity based on seven disomic loci ranged from 0.27 to 0.46 and 0.21 to 0.51, respectively. The average Shannon index ranged from 0.26 to 0.94 in eight nondisomic loci. Conclusions: The SSR markers described here may be useful for further investigation of population genetics and invasion dynamics of S. alterniflora.

Convergent adaptive evolution in marginal environments: unloading transposable elements as a common strategy among mangrove genomes

Several clades of mangrove trees independently invade the interface between land and sea at the margin of woody plant distribution. As phenotypic convergence among mangroves is common, the possibility of convergent adaptation in their genomes is quite intriguing. To study this molecular convergence, we sequenced multiple mangrove genomes. In this study, we focused on the evolution of transposable elements (TEs) in relation to the genome size evolution. TEs, generally considered genomic parasites, are the most common components of woody plant genomes. Analyzing the long terminal repeat-retrotransposon (LTR-RT) type of TE, we estimated their death rates by counting solo-LTRs and truncated elements. We found that all lineages of mangroves massively and convergently reduce TE loads in comparison to their nonmangrove relatives; as a consequence, genome size reduction happens independently in all six mangrove lineages; TE load reduction in mangroves can be attributed to the paucity of young elements; the rarity of young LTR-RTs is a consequence of fewer births rather than access death. In conclusion, mangrove genomes employ a convergent strategy of TE load reduction by suppressing element origination in their independent adaptation to a new environment.

Transcriptome analyses provide insights into the phylogeny and adaptive evolution of the mangrove fern genus Acrostichum.

The mangrove fern genus Acrostichum grows in the extremely unstable marine intertidal zone under harsh conditions, such as high salt concentrations, tidal rhythms and long-term climate changes. To explore the phylogenetic relationships and molecular mechanisms underlying adaptations in this genus, we sequenced the transcriptomes of two species of Acrostichum, A. aureum and A. speciosum, as well as a species in the sister genus, Ceratopteris thalictroides. We obtained 47,517, 36,420 and 60,823 unigenes for the three ferns, of which 24.39–45.63% were annotated using public databases. The estimated divergence time revealed that Acrostichum adapted to the coastal region during the late Cretaceous, whereas the two mangrove ferns from the Indo West-Pacific (IWP) area diverged more recently. Two methods (the modified branch-site model and the Kh method) were used to identify several positively selected genes, which may contribute to differential adaptation of the two Acrostichum species to different light and salt conditions. Our study provides abundant transcriptome data and new insights into the evolution and adaptations of mangrove ferns in the inhospitable intertidal zone.

Extremely low genetic diversity across mangrove taxa reflects past sea level changes and hints at poor future responses

The projected increases in sea levels are expected to affect coastal ecosystems. Tropical communities, anchored by mangrove trees and having experienced frequent past sea level changes, appear to be vibrant at present. However, any optimism about the resilience of these ecosystems is premature because the impact of past climate events may not be reflected in the current abundance. To assess the impact of historical sea level changes, we conducted an extensive genetic diversity survey on the Indo-Malayan coast, a hotspot with a large global mangrove distribution. A survey of 26 populations in six species reveals extremely low genome-wide nucleotide diversity and hence very small effective population sizes (Ne ) in all populations. Whole-genome sequencing of three mangrove species further shows the decline in Ne to be strongly associated with the speed of past changes in sea level. We also used a recent series of flooding events in Yalong Bay, southern China, to test the robustness of mangroves to sea level changes in relation to their genetic diversity. The events resulted in the death of half of the mangrove trees in this area. Significantly, less genetically diverse mangrove species suffered much greater destruction. The dieback was accompanied by a drastic reduction in local invertebrate biodiversity. We thus predict that tropical coastal communities will be seriously endangered as the global sea level rises. Well-planned coastal development near mangrove forests will be essential to avert this crisis.