Phenology and polyploidy in annualBrachypodiumspecies (Poaceae) along the aridity gradient in Israel

Abstract

Local adaptation of plants along environmental gradients provides strong evidence for clinal evolution mediated by natural selection. Plants have developed diverse strategies to mitigate stress, for example, drought escape is a phenological strategy to avoid drought stress, while polyploidy was proposed as a genomic adaptation to stress. Polyploidy as an adaptation to aridity (an environmental parameter integrating temperature and precipitation) was previously documented in annual Brachypodium spp. (Poaceae) in the Western Mediterranean. Here, we examined whether polyploidy or phenology are associated with aridity in annual Brachypodium spp. along the aridity gradient in the Eastern Mediterranean. Using flow cytometry, we determined ploidy levels of plants from natural populations along the Israeli gradient, spanning ∼424 km from mesic Mediterranean to extreme desert climates. In a common garden we recorded time of seedling emergence, flowering and senescence. We tested whether the proportion of allotetraploids in the populations and phenological traits were associated with aridity. Contrary to a previous study in the Western Mediterranean, we found no effect of aridity on the proportion of allotetraploids and diploids within populations. Interestingly, phenology was associated with aridity: time of emergence was later, while flowering and senescence were earlier in desert plants. Our results indicate that in the Eastern Mediterranean, adaptation of Brachypodium to aridity is mediated mainly by phenology, rather than ploidy level. Therefore, we suggest that genome duplication is not the main driver of adaptation to environmental stress; rather, phenological change as a drought escape mechanism may be the major adaptation.

Local adaptation to abiotic and biotic stresses and phenotypic selection on flowering time in annual Brachypodium spp. along an aridity gradient

“…For climatic characterization of each population, mean annual precipitation and mean annual temperature data were retrieved from the Israeli meteorological service website (http://ims.gov.il). From these data we calculated an aridity index as the ratio between the mean annual precipitation and temperature (Manzaneda et al ., 2012; Penner et al ., 2019).…”

“…Plants of the annual Brachypodium species complex (formerly known as one species, B. distachyon ; Catalán et al ., 2012) are self–fertile grasses (Vogel et al ., 2009), distributed around the Mediterranean basin and in western Asia, with a wide ecological niche (Opanowicz et al ., 2008; López-Alvarez et al ., 2012), including along steep climatic gradients both in Israel (Kigel et al ., 2011; Bareither et al ., 2017; Penner et al ., 2019) and in the Iberian peninsula (Manzaneda et al ., 2012). The annual Brachypodium complex was recently split into three different karyotypic species (López-Alvarez et al ., 2012; López-Alvarez et al ., 2015): B. distachyon (diploid; 2n=10), B. stacei (diploid; 2n=20), and B. hybridum (allotetraploid; 2n=30) (Catalán et al ., 2012).…”

“…from the Iberian Peninsula showed that increased genome size by polyploidy is associated with aridity (Manzaneda et al ., 2012), but two recent studies found no such association between genome size and aridity along the Israeli aridity gradient (Bareither et al ., 2017; Penner et al ., 2019). This lack of an association suggests that genome size is not a mechanism that drives adaptation to climate (Bareither et al ., 2017; Penner et al ., 2019). Given no genomic adaptation, and being indistinguishable morphologically in Israel (Penner et al ., 2019), we consider all annual Brachypodium cytotypes in Israel as one taxon.…”

“…This lack of an association suggests that genome size is not a mechanism that drives adaptation to climate (Bareither et al ., 2017; Penner et al ., 2019). Given no genomic adaptation, and being indistinguishable morphologically in Israel (Penner et al ., 2019), we consider all annual Brachypodium cytotypes in Israel as one taxon. While genome duplication is not the driver of adaptation to climate in Brachypodium , phenological variation correlates with aridity suggest that the adoption of an escape strategy by earlier flowering in arid conditions provides Brachypodium the ability to grow along steep aridity gradients (Kigel et al ., 2011; Penner et al ., 2019).…”

“…Given no genomic adaptation, and being indistinguishable morphologically in Israel (Penner et al ., 2019), we consider all annual Brachypodium cytotypes in Israel as one taxon. While genome duplication is not the driver of adaptation to climate in Brachypodium , phenological variation correlates with aridity suggest that the adoption of an escape strategy by earlier flowering in arid conditions provides Brachypodium the ability to grow along steep aridity gradients (Kigel et al ., 2011; Penner et al ., 2019). The availability of plants from a wide natural distribution along the entire aridity gradient in Israel, together with the ease of growing this annual grass, offers an opportunity to test for selection regimes that may drive adaptation to climate and biotic interactions in populations of annual Brachypodium .…”