DOI: 10.5061/dryad.74jt1
title: Data from: Seasonal changes in pollen limitation and femaleness along the snowmelt gradient in a distylous alpine herb, Primula modesta

description: Flowering phenology of alpine plants is strongly determined by the timing
 of snowmelt, and the conditions of pollination of widely distributed
 plants vary greatly during their flowering season. We examined the
 reproductive success of the distylous alpine herb, Primula modesta, along
 the snowmelt gradient under natural conditions, and compared it with the
 result of artificial pollination experiments. In addition, the
 compositions and visit frequencies of pollinators to the flower of P.
 modesta were examined during the flowering period. The pin and thrum
 plants of P. modesta growing at the same site have an equal ability to
 produce seeds if a sufficient amount of legitimate pollen grains are
 deposited on the stigma surface. However, under natural conditions, their
 seed-set success was often (even if not always) restricted by pollen
 limitation, and the functional gender of the pin and thrum plants biased
 to the female and male, respectively, associated with their growing sites.
 These variations were not ascribed to resource limitation nor biased morph
 ratio but to the seasonal changes in pollination situations, a replacement
 of pollinator types from long- to short-tongued pollinators resulted in
 unidirectional pollen transfer from long stamens (thrum plants) to long
 styles (pin plants). The functional gender specialization may enhance the
 evolution of dioecy from heterostyly, but the severe pollen limitation may
 cause the breakdown of heterostyly into homostyly. To consider the
 evolutionary pathway of heterostylous plants, an accumulation of the
 empirical data is required demonstrating how phenological synchrony
 between plants and pollinators is decided and to what degree this
 relationship is stable over years, along with estimates of selection and
 gene flow in individual plants.
description: Number of flowers per
 plantWe set 14, 4, and 5
 quadrats of the size 0.5 m × 0.5 m at the ridges, slopes, and valleys,
 respectively. During 2009 and 2010, we counted the number of flowering
 plants, flowers per plant, and total number of flowers within each
 quadrat, discriminating between morphs (pin and thrum).Number of
 plantsWe set 14, 4, and 5
 quadrats of the size 0.5 m × 0.5 m at the ridges, slopes, and valleys,
 respectively. During 2009 and 2010, we counted the number of plants within
 each quadrat.Pollinator
 visitationWe observed pollinator
 visits to the flowers of P. modesta during peak flowering at each site:
 early June, early July, and late July at the ridge, slope, and valley
 sites, respectively. Field observations were conducted during calm and
 fine periods during daylight hours (9:00 am–3:00 pm). The total
 observation times were 25 hours per 5 days, 12 hours per 3 days, and 14
 hours per 4 days, at the ridge, slope, and valley sites, respectively. On
 each observation day, we arbitrary set 2 m × 2 m plot and recorded the
 pollinators and the number of flowers of P. modesta. Pollinators were
 classified into Diptera, Hymenoptera, Lepidoptera, and others during field
 observations, and major pollinators were collected and identified at the
 family or species level if possible.Seed-set by experimental
 crossesTo estimate the
 potential ability to produce seeds at each site (ridge, slope, and
 valley), we conducted artificial pollination experiments within and nearby
 the quadrats over 2 years (2010 and 2011). The total numbers of pin and
 thrum plants pollinated were 78 and 77 during 2010 and 59 and 81 during
 2011, respectively. The plants were covered with fine-meshed nylon nets to
 exclude insect visitors. For each plant, one target flower bud was
 emasculated just before opening, and other flowers were removed. After
 opening of the target flowers, hand pollination was conducted by
 legitimate pollen grains collected from a single pollen donor growing at
 least 5 m away from the recipient plant. All fruits derived from the
 pollination treatments were harvested just before dehiscence, and the
 number of seeds and undeveloped seeds or ovules were counted using a
 microscope.Seed-set under natural
 conditionsTo estimate the number
 of ovules and the seeds per fruit under natural conditions at each site
 (ridge, slope, and valley), we examined the plants growing within and
 nearby the quadrats over 2 years (2010 and 2011). The total number of pin
 and thrum plants examined was 78 and 79 during 2010 and 170 and 157 during
 2011, respectively. We collected several fruits from each plant (2.8 on
 average) during 2010, and one fruit per plant during 2011. The total
 number of fruits collected from the pin and thrum plants was 233 and 204
 during 2010 and 170 and 157 during 2011, respectively. The fruits were
 collected just before dehiscence, and the number of seeds and undeveloped
 seeds or ovules were counted using a microscope.