Self-incompatibility of U.S. Great Basin Sphaeralcea (Malvaceae) and regional prevalence, pollination efficacy, and floral infidelity of their oligolectic Diadasia bees

Abstract

Three globemallows -- Sphaeralcea coccinea, S. grossularifolia and S. munroana (Malvaceae) -- are widespread, summer-flowering perennial forbs found throughout the sagebrush-steppe and neighbouring biomes of the western USA. Their seed is desirable for rehabilitating degraded rangelands. This study had four objectives: 1) determine each species’ breeding biology, 2) quantitatively characterize their regional pollinator guilds, and evaluate 3) pollination efficacy and 4) floral constancy of a bee that is an abundant Sphaeralcea specialist.

All three globemallows are self-incompatible. On average, 43-76% of manually outcrossed flowers formed capsules (schizocarps) averaging 7-9 seeds each (potential of 11 seeds). In contrast, autogamy and geitonogamy yielded few capsules, half of them seedless. Autogamy yielded few mature seeds despite proximity of anthers and stigmas.

Floral visitors to S. grossularifolia and S. munroana were systematically surveyed across the American sagebrush-steppe. Bees foraging at wild and cultivated populations were non-social ground-nesters. Three globemallow pollen specialists (oligoleges, particularly Diadasia diminuta) were frequent and often abundant, as were several common generalist halictid bees among the 38 species collected. Single floral visits by pollen-foraging females of D. diminuta deposited sufficient pollen to effect full seed set (means of 42 and 34 grains per stigma at S. coccinea and S. grossularifolia, respectively).

Newly planted Sphaeralcea plots were colonized within several years by D. diminuta. Females foraging a mixed experimental garden frequently switched between globemallow species during foraging bouts, evidence that these floral specialists will facilitate hybridization rather than behavioral isolation of Sphaeralcea. The abundant pollinators at these Sphaeralcea species should deliver sufficient outcrossing at most locales, including both wildlands and farmed fields.