Increasing pollen production at high latitudes across animal-pollinated flowering plants

Aim: Plant reliance on animal mutualists is expected to decrease with
latitude owing to increasing environmental instability. As a consequence,
more erratic animal pollination in the temperate zones than in the tropics
could translate into lower efficiency in pollen transfer, and thus
increasing pollen wastage. Despite the relevance of this hypothesis for
plant reproductive evolution, the implications of a proposed latitudinal
gradient in pollinator reliability for pollen and ovule production, traits
directly affecting seed siring and seed set, respectively, remain
unresolved. Location: Global. Time period:1971-2020. Major taxa studied:
Angiospermae. Methods: Based on a bibliographic survey and our own data,
we collated a dataset with information on pollen production (P) and ovule
number (O) per flower from 419 studies, including a total of 1392
animal-pollinated angiosperm species from 141 families distributed
worldwide and sampled between the equator and 68.35o latitude. Using
phylogenetic general linear mixed models, we investigated latitudinal
variation in P and O. We also tested a latitudinal effect on several
ancillary plant traits associated with a plant’s mating system and
pollinator specialization that might confound any latitudinal effect on
gamete production. Results: P but not O was positively associated with
latitude, a trend that even became stronger after including the
latitudinally-varying ancillary traits (i.e., growth form, flower size,
flower symmetry and number of pollinator orders). The latitudinal effect
on P was more pronounced among species producing large flowers, a trait
that could be linked to self-incompatibility and thus outcrossing. A weak
phylogenetic patterning of P also indicates high evolvability potential of
this trait that can have an effect on both male and female plant fitness.
Main conclusions: Even though a latitudinal trend in pollinator
reliability still awaits direct testing, the observed latitudinal increase
in P across angiosperms can be interpreted as an evolutionary consequence
of an increasingly unpredictable pollination environment.