Sex determination in gymnosperms

Intraspecific and sex-dependent variation of leaf traits along altitude gradient in the endangered dioecious tree Taxus fuana Nan Li & R.R. Mill

Plant intraspecific trait variation (ITV) including sex-dependent differences are matters of many ecological consequences, from individual to ecosystem, especially in endangered and rare species. Taxus fuana is an endangered dioecious species with small and isolated populations endemic to the Himalayas region. Little is known about its trait variation between sexes, and among populations. In this study, 18 leaf traits from 179 reproductive trees (males and females) along the altitude (2600-3200m a.s.l.) of the T. fuana populations distributed in Gyirong County, Tibet, China, were measured. ITV and sources of variation in leaf traits were assessed. The relationship between leaf traits of males and females and altitude was analyzed separately. Variations in leaf traits of T. fuana ranged from 3.1% to 24.2%, with the smallest in leaf carbon content and the largest in leaf thickness to area ratio. On average 78.13% of the variation in leaf traits was from within populations and 21.87% among populations. The trends in leaf width, leaf nitrogen to phosphorus ratio, leaf carbon to nitrogen ratio, leaf carbon isotope ratio, and leaf nitrogen isotope ratio in relation to altitude were the same for males and females. Leaf length to width ratio varied significantly with altitude only in males, while leaf phosphorus content, leaf nitrogen content, and leaf carbon to phosphorus ratio varied significantly with altitude only in females. The correlation coefficients of most leaf traits of females with altitude were larger than that of males. In the relationship between leaf traits, there was a high similarity among males and females, but the altitude accounted for more explanation in females than in males. Our results suggested that the variation in leaf traits of T. fuana was small and did not dominate the interspecific competition in the local communities. Adaptation to the altitude gradient of T. fuana might be through altering nutrient storage processes and water use efficiency. Adaptation of male and female T. fuana to environmental changes showed differences, where the males were more tolerant and the females responded greatly to altitude. The differences in adaptation strategies between male and female T. fuana may be detrimental to the maintenance of their populations.

Evolution of the sex-determining region in Ginkgo biloba

Sex chromosomes or sex-determining regions (SDR) have been discovered in many dioecious plant species, including the iconic 'living fossil' Ginkgo biloba, though the location and size of the SDR in G. biloba remain contradictory. Here we resolve these controversies and analyse the evolution of the SDR in this species. Based on transcriptome sequencing data from four genetic crosses we reconstruct male- and female-specific genetic maps and locate the SDR to the middle of chromosome 2. Integration of the genetic maps with the genome sequence reveals that recombination in and around the SDR is suppressed in a region of about 50 Mb in both males and females. However, occasional recombination does occur except a small, less than 5 Mb long region that does not recombine in males. Based on synonymous divergence between homologous X- and Y-linked genes in this region, we infer that the Ginkgo SDR is fairly old-at least of Cretaceous origin. The analysis of substitution rates and gene expression reveals only slight Y-degeneration. These results are consistent with findings in other dioecious plants with homomorphic sex chromosomes, where the SDR is typically small and evolves in a region with pre-existing reduced recombination, surrounded by long actively recombining pseudoautosomal regions. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.