A 313 plastome phylogenomic analysis of Pooideae: Exploring relationships among the largest subfamily of grasses.

In this study, we analyzed 313 plastid genomes (plastomes) of Poaceae with a focus on expanding our current knowledge of relationships among the subfamily Pooideae, which represented over half the dataset (164 representatives). In total, 47 plastomes were sequenced and assembled for this study. This is the largest study of its kind to include plastome-level data, to not only increase sampling at both the taxonomic and molecular levels with the aim of resolving complex and reticulate relationships, but also to analyze the effects of alignment gaps in large-scale analyses, as well as explore divergences in the subfamily with an expanded set of 14 accepted grass fossils for more accurate calibrations and dating. Incorporating broad systematic assessments of Pooideae taxa conducted by authors within the last five years, we produced a robust phylogenomic reconstruction for the subfamily, which included all but two supergeneric taxa (Calothecinae and Duthieeae). We further explored how including alignment gaps in plastome analyses oftentimes can produce incorrect or misinterpretations of complex or reticulate relationships among taxa of Pooideae. This presented itself as consistently changing relationships at specific nodes for different stripping thresholds (percentage-based removal of gaps per alignment column). Our summary recommendation for large-scale genomic plastome datasets is to strip alignment columns of all gaps to increase pairwise identity and reduce errant signal from poly A/T bias. To do this we used the "mask alignment" tool in Geneious software. Finally, we determined an overall divergence age for Pooideae of roughly 84.8 Mya, which is in line with, but slightly older than most recent estimates.

Zygnema circumcarinatum UTEX 1559 chloroplast and mitochondrial genomes provide insight into land plant evolution.

The complete chloroplast and mitochondrial genomes of Charophyta have shed new light on land plant terrestrialization. Here, we report the organellar genomes of the Zygnema circumcarinatum strain UTEX 1559, and a comparative genomics investigation of 33 plastomes and 18 mitogenomes of Chlorophyta, Charophyta (including UTEX 1559 and its conspecific relative SAG 698-1a), and Embryophyta. Gene presence/absence was determined across these plastomes and mitogenomes. A comparison between the plastomes of UTEX 1559 (157 548 bp) and SAG 698-1a (165 372 bp) revealed very similar gene contents, but substantial genome rearrangements. Surprisingly, the two plastomes share only 85.69% nucleotide sequence identity. The UTEX 1559 mitogenome size is 215 954 bp, the largest among all sequenced Charophyta. Interestingly, this large mitogenome contains a 50 kb region without homology to any other organellar genomes, which is flanked by two 86 bp direct repeats and contains 15 ORFs. These ORFs have significant homology to proteins from bacteria and plants with functions such as primase, RNA polymerase, and DNA polymerase. We conclude that (i) the previously published SAG 698-1a plastome is probably from a different Zygnema species, and (ii) the 50 kb region in the UTEX 1559 mitogenome might be recently acquired as a mobile element.

Plastome phylogenomics and characterization of rare genomic changes as taxonomic markers in plastome groups 1 and 2 Poeae (Pooideae; Poaceae).

A phylogenomic analysis of 42 complete plastid genomes (plastomes), including 16 that were newly sequenced, was conducted. Plastomes were sampled from 19 subtribes of Pooideae, to investigate relationships within and between Chloroplast Group 1 (Aveneae) and Group 2 (Poeae) species. Two data partitions: complete plastomes, and a combined plastome and rare genomic change (RGC) data matrix, were analyzed. Overall, 156 non-ambiguous RGC were identified, of which homology was inferred for 38 RGC. Among the 38 RGC identified, six were synapomorphic among the Group 1 subtribes: Aveninae, Agrostidinae, and Anthoxanthinae, (Phalaridinae + Torreyochloinae), and 27 were synapomorphic among the Group 2 subtribes: Loliinae, (Ammochloinae + Parapholiinae + Dactylidinae), Parapholiinae, Dactylidinae, Poinae, and Coleanthinae. Four RGC were determined to be homoplasious in Groups 1 and 2. Two other RGC originated through intrastrand deletion events. The remaining RGC events likely originated through recombination given their size and lack of sequence evidence for other types of mutations. This study also determined that relationships between taxa, even those only weakly supported in previous studies, could be inferred with strong support when utilizing complete plastomes.

Plastid phylogenomic study of species within the genus Zea: rates and patterns of three classes of microstructural changes.

This project examines the relationships within the genus Zea using complete plastid genomes (plastomes). While Zea mays has been well studied, congeneric species have yet to be as thoroughly examined. For this study four complete plastomes and a fifth nearly complete plastome were sequenced in the five species (Zea diploperennis, Zea perennis, Zea luxurians, Zea nicaraguensis, and Zea mays subsp. huehuetenangensis) by Sanger or next-generation methods. An analysis of the microstructural changes, such as inversions, insertion or deletion mutations (indels) and determination of their frequencies were performed for the complete plastomes. It was determined that 193 indels and 15 inversions occurred across the examined plastomes of Zea. Tandem repeat indels were the most common type of microstructural change observed. Divergence times were estimated using a noncorrelated relaxed clock method. Divergence dates for specific nodes relative to Zea were calculated to fall between 38,000 years before present (YBP) for the subspecies included in this study and 23,000 YBP for section Luxuriantes included in this study. The stem lineage of all Zea species was calculated to have diverged at 176,000 YBP. The calculated mutation rates for the genus fell within the range of 1.7E-8 to 3.5E-8 microstructural changes per site per year. These rates of change are not uniform, despite the close relationships of taxa in this study. Phylogenomic analyses using full plastome alignments were also conducted to compare tree topologies from different types of mutations. In most cases, the previous work examining Zea mitochondrial and nuclear data was confirmed.