Rapid, cost-effective and accurate quantification of Yucca schidigera Roezl. steroidal saponins using HPLC-ELSD method

Yucca GRAS-labelled saponins have been and are increasingly used in food/feed, pharmaceutical or cosmetic industries. Existing techniques presently used for Yucca steroidal saponin quantification remain either inaccurate and misleading or accurate but time consuming and cost prohibitive. The method reported here addresses all of the above challenges. HPLC/ELSD technique is an accurate and reliable method that yields results of appropriate repeatability and reproducibility. This method does not over- or under-estimate levels of steroidal saponins. HPLC/ELSD method does not require each and every pure standard of saponins, to quantify the group of steroidal saponins. The method is a time- and cost-effective technique that is suitable for routine industrial analyses. HPLC/ELSD methods yield a saponin fingerprints specific to the plant species. As the method is capable of distinguishing saponin profiles from taxonomically distant species, it can unravel plant adulteration issues.

Floral scent of Joshua trees (Yucca brevifolia sensu lato): Divergence in scent profiles between species but breakdown of signal integrity in a narrow hybrid zone

PREMISE OF THE STUDY

The role of floral scent in facilitating reproductive isolation between closely related plants remains poorly understood. Yucca brevifolia and Yucca jaegeriana are pollinated by different moth species in allopatry, but in a narrow contact zone, pollinator-host specificity breaks down, resulting in hybridization between species. We explored the chemical basis for reproductive isolation and hybridization in these Joshua trees by characterizing the floral scent of each species in allopatry, analyzing scent profiles from trees in the contact zone, and matching these data with genotypic and phenotypic data.

METHODS

We analyzed floral volatiles using gas chromatography-mass spectrometry, tested for species divergence of scent profiles and classified trees in the contact zone as hybrid or either parental species. We used floral and vegetative morphological data and genotypic data to classify trees and analyzed whether certain trait combinations were more correlated than others with respect to assignment of trees and whether frequencies of classified tree types differed depending on which data set was used.

KEY RESULTS

The Joshua tree floral scent included oxygenated 8-carbon compounds not reported for other yuccas. The two species differed (P < 0.001) in scent profiles. In the contact zone, many hybrids were found, and phenotypic traits were generally weakly correlated, which may be explained by extensive gene flow between species or by exposure to different selection pressures.

CONCLUSIONS

Although the two Joshua tree species produce distinct floral scent profiles, it is insufficient to prevent attraction of associated pollinators to both hosts. Instead, floral morphology may be the key trait mediating gene flow between species.

Population genomics of divergence within an obligate pollination mutualism: Selection maintains differences between Joshua tree species

PREMISE OF THE STUDY

Speciation is a complex process that can be shaped by many factors, from geographic isolation to interspecific interactions. In Joshua trees, selection from pollinators on style length has been hypothesized to contribute to the maintenance of differentiation between two hybridizing sister species. We used population genomics approaches to measure the extent of genetic differentiation between these species, test whether selection maintains differences between them, and determine whether genetic variants associated with style length show signatures of selection.

METHODS

Using restriction-site-associated DNA (RAD)-sequencing, we identified 9516 single nucleotide polymorphisms (SNPs) across the Joshua tree genome. We characterized the genomic composition of trees in a narrow hybrid zone and used genomic scans to search for signatures of selection acting on these SNPs. We used a genome-wide association study to identify SNPs associated with variation in phenotypic traits, including style length, and asked whether those SNPs were overrepresented among the group under selection.

KEY RESULTS

The two species were highly genetically differentiated (F_ST = 0.25), and hybrids were relatively rare in the hybrid zone. Approximately 20% of SNPs showed evidence of selection maintaining divergence. While SNPs associated with style length were overrepresented among those under selection (P << 0.0001), the same was true for SNPs associated with highly differentiated vegetative traits.

CONCLUSIONS

The two species of Joshua tree are clearly genetically distinct, and selection is maintaining differences between them. We found that loci associated with differentiated traits were likely to be under selection. However, many traits other than style length appeared to be under selection. Together with the dearth of intermediate hybrids, these findings reveal that these taxa are more strongly diverged than previously suspected and that selection, likely on many targets, is maintaining separation where the two species meet and hybridize.

Yucca brevifolia fruit production, predispersal seed predation, and fruit removal by rodents during two years of contrasting reproduction

PREMISE OF THE STUDY

The distribution of Yucca brevifolia, a keystone species of the Mojave Desert, may contract with climate change, yet reproduction and dispersal are poorly understood. We tracked reproduction, seed predation, and fruit dispersal for two years and discuss whether Y. brevifolia is a masting species.

METHODS

Fruit maturation, seed predation (larval yucca moths), and fruit dispersal (rodents) were monitored on a random sample of panicles during 2013 and 2014, which were years of high and low reproduction, respectively. Fates of fruits placed on the ground and in canopies were also tracked. Rodents were live-trapped to assess abundance and species composition.

KEY RESULTS

In 2013, 66% of inflorescences produced fruit of which 53% escaped larval predation; 19.5% of seeds were destroyed in infested fruits. Total seed production was estimated to be >100 times greater in 2013 than 2014. One-third of the fruit crop fell to the ground and was removed by rodents over the course of 120 d. After ground fruits became scarce, rodents exploited canopy fruits. Rodent numbers were low in 2013, so fruits remained in canopies for 370 d. In 2014, fruit production was approximately 20% lower. Larvae infested the majority of fruits, and almost twice the number of seeds were damaged. Fruits were exploited by rodents within 65 d.

CONCLUSIONS

High fertilization, prolific seed production, and low predispersal predation in 2013 suggests that pollinator attraction and satiation of seed predators influence masting in Y. brevifolia. Abundant, prolonged fruit availability to seed-dispersing rodents likely extends recruitment opportunities during mast years.