Reproductive isolation and the maintenance of species boundaries in two serpentine endemic Jewelflowers

Abstract

Speciation occurs when reproductive barriers substantially reduce gene flow between lineages. Understanding how specific barriers contribute to reproductive isolation offers insight into the initial forces driving divergence and the evolutionary and ecological processes responsible for maintaining diversity. Here, we quantified multiple pre‐ and post‐pollination isolating barriers in a pair of closely related California Jewelflowers (Streptanthus, Brassicaceae) living in an area of sympatry. S. breweri and S. hesperidis are restricted to similar serpentine habitats; however, populations are spatially isolated at fine‐scales and rarely co‐occur in intermixed stands. Several intrinsic postzygotic barriers were among the strongest we quantified, yet, postzygotic barriers currently contribute little to overall reproductive isolation due to the cumulative strength of earlier‐acting extrinsic barriers, including spatial isolation, and flowering time and pollinator differences. Data from multiple years suggest that pre‐pollination barriers may have different strengths depending on annual environmental conditions. Similarly, crossing data suggest that the strength of intrinsic isolation may vary among different population pairs. Estimates of total reproductive isolation in S. breweri and S. hesperidis are robust to uncertainty and variability in individual barrier strength estimates, demonstrating how multiple barriers can act redundantly to prevent gene flow between close relatives living in sympatry.

Phenotypic and genotypic parallel evolution in parapatric ecotypes ofSenecio

“…We may be unknowingly biased towards measuring traits that are divergent between ecotypes, rather than the overall phenotype of an organism. We might also overlook parallelism in traits we did not measure, such as adaptive phenotypes like flowering time in plants (Blackman et al, 2011;Christie & Strauss, 2019), or disregard traits that would be difficult to measure (such as the root networks of plants), or physiological responses to a stimulus (such as tropisms in plants).…”

Frequency-dependent fitness and reproductive dynamics contribute to habitat segregation in sympatric jewelflowers

“…are restricted to rocky, isolated serpentine outcrops in the interior Coast Range of California, where they share an area of geographic overlap [56]. Previous field surveys in an area of sympatry indicate that the two species rarely occur in intermixed stands, but instead are spatially isolated in single-species patches on the same, or closely adjacent (less than 100 m), serpentine outcrops [57]. Twenty-six of 32 previously identified patches within a 7000-acre area consisted of a single species, while only three patches of each species were intermixed [57].…”

“…Hybrid crosses suffer reduced seed production (with S. breweri as maternal parents) and F 1 s show reduced viability and fertility (in both directions of the cross). Heterospecific pollen can effectively fertilize both species resulting fruit and seed production [57], however HPT leads to reduced seed viability (electronic supplementary material, Experimental pollination). Both species also suffer significantly reduced seed viability (61-71% viable) when receiving 50 : 50 mixed pollen compared to when receiving pure conspecific pollen (97-100% viable; electronic supplementary material, figure S1).…”

Natural selection maintains species despite widespread hybridization in the desert shrubEncelia

“…To accommodate this discrepancy, many speciation models require prezygotic barriers to evolve quickly after adaptation has been initiated in order to "complete" speciation by locking in locally adaptive traits (7,8) . For example, prezygotic isolating mechanisms such as assortative mating, shifts in phenology, and geographic isolation are frequently invoked as operating alongside postzygotic isolating mechanisms, such as selection against hybrids, presumably because prezygotic sources of isolation are thought to be difficult to reverse (9)(10)(11) . Some systems, however, do not fit this paradigm and species remain distinct despite exhibiting little evidence for assortative mating and little geographic or phenological isolation.…”