4 Search Results

Sex chromosomes have evolved hundreds of times across the flowering plant tree of life; their recent origins in some members of this clade can shed light on the early consequences of suppressed recombination, a crucial step in sex chromosome evolution. Amborella trichopoda, the sole species of a lineage that is sister to all other extant flowering plants, is dioecious with a young ZW sex determination system. Here we present a haplotype-resolved genome assembly, including highly contiguous assemblies of the Z and W chromosomes. We identify a ~3-megabase sex-determination region (SDR) captured in two strata that includes a ~300-kilobase inversion that is enriched with repetitive sequences and contains a homologue of the Arabidopsis METHYLTHIOADENOSINE NUCLEOSIDASE (MTN1-2) genes, which are known to be involved in fertility. However, the remainder of the SDR does not show patterns typically found in non-recombining SDRs, such as repeat accumulation and gene loss. These findings are consistent with the hypothesis that dioecy is derived in Amborella and the sex chromosome pair has not significantly degenerated.

The evolution of plastid genomes (plastomes) in land plants is typically conservative, with extensive structural rearrangements present in only a few groups. Early Southern blot analysis identified two Lobelia species that minimally required deletion of the plastid gene accD and five inversions to account for their plastome arrangement relative to the ancestral organization. Sixty alternative 5-step inversion scenarios could account for the observed arrangement, but only one scenario was consistent with the criterion of ‘common cause’ attributable to a putative rearrangement hot spot at the accD deletion-site. Plastome sequencing demonstrated that this previously hypothesized inversion order is historically accurate. Detailed reconstructions of the ancestral plastome organization before and after each inversion are presented herein. Stem-loop and disruption-rescue models were evaluated for each inversion. One inversion has an obvious stem-loop basis, but the other four inversions were primarily caused by serial insertion of foreign (extra-plastid) DNA bearing large open-reading frames that disrupted plastome organization at the accD deletion-site, and complete plastomes were rescued by seemingly arbitrary ligation or fortuitous recombination at the other inversion endpoint. Transposed copies of DNA segments from elsewhere in the plastome are frequently inserted at inversion junctions, and four junctions are consistent with the stem-loop ligation model.

Variation in population sex ratio is particularly pronounced in gynodioecious angiosperms. Extremely high female frequencies in gynodioecious populations cannot be readily explained by selective forces alone. To assess the contributions of drift and gene flow to extreme sex-ratio variation, we documented sex ratio and population size in 92 populations of Lobelia siphilitica across its range and genotyped plants using plastid and nuclear genetic markers. Similarity in spatial patterns of genetic and demographic variables may suggest that drift and/or gene flow have contributed to population sex-ratio variation in L. siphilitica. We found strong spatial structuring of extremely high female frequencies: populations with >50% female plants are restricted to the south–central portion of the range. However, we did not detect any spatial structuring in population size nor metrics of genetic diversity, suggesting that extreme variation in female frequency is not strongly affected by drift or gene flow. Extreme sex-ratio variation is frequently observed in gynodioecious plants, but its causes are difficult to identify. Further investigation into mechanisms that create or maintain the spatial structure of sex ratios in gynodioecious species will provide much needed insight.

PREMISE OF THE STUDY: Rapid radiations are difficult to reconstruct when organismal diversification and biogeographic movement outpace the evolution of genes typically used in phylogenetic analyses. The 125 kb of unique sequence from complete plastid genomes (= plastomes) largely solves the molecular sampling problem, and taxon sampling that triangulates the base of each major subclade largely solves the long‐branch attraction problem. This combination of molecular and phylogenetic sampling is used to reconstruct the cosmopolitan radiation of lobeliads, with special focus on the origin of the giant lobelias. METHODS: An alignment of 18 previously generated and 61 new plastomes was analyzed to produce the phylogenetic estimate upon which the biogeographic reconstruction was based. KEY RESULTS: Originating in southern Africa, the Lobeliaceae underwent a spectacular cosmopolitan radiation about 20 million years ago. One lineage colonized Madagascar and eastern Asia, which was the source area for the evolution of the giant lobelias. A second lineage colonized the Mediterranean and North America, in quick succession. South America and Australia were also colonized from South Africa, most likely as independent events, but detailed biogeographic reconstruction is limited by inferred extinction events. The south Pacific segregate genera Apetahia and Sclerotheca are inferred to have Hawaiian ancestry. The East African radiation independently reached Ethiopia, West Africa, and Brazil. CONCLUSIONS: With adequate molecular and taxon sampling, many details of rapid radiations can be accurately inferred. However, not all lineages survived, and analyses of extant species cannot recover details that have been lost due to extinction.