DOI: 10.5061/dryad.kwh70rz6z
title: Variability in mating strategies in a wide spread cactus in the Chihuahuan desert

description: Opuntia is the most diverse and widespread genus in the Cactaceae; several
 species have broad distribution ranges and have successfully adapted to
 distinct habitats. One explanation for their success is the diversity of
 their reproductive systems. Opuntia macrocentra Engelm is one of the most
 common widespread species in North American deserts. Previous work on a
 northern population documented that this species has a mixed-mating system
 and receives a low frequency of pollinators. In a southern population of
 O. macrocentra, we described its floral visitors, anthesis schedules, and
 flower morphometry. Through controlled pollination experiments (selfing,
 outcrossing, supplementary pollen, and natural pollination), we assessed
 the breeding and mating systems and measured pollen limitation. Eight
 different bee species visited the flowers. Based on their conduct,
 Diadasia rinconis and Lasioglossum sp. were the main pollinators. Flowers
 were homogamous and herkogamous. Floral traits and pollination experiments
 showed that O. macrocentra is xenogamous and self-incompatible. The fruit
 set of the natural pollination, outcrossing, and supplementary pollen
 treatments were not statistically different. The seed set was higher in
 natural pollination and the pollen limitation index for the seed set was
 negative. Based on these results, we conclude that O. macrocentra is not
 pollen limited in the studied population. These results contrast with
 those of the previously studied population; therefore, we caution about
 making generalizations about the reproductive system based solely on
 studies in one population. The diversity of mating systems displayed by O.
 macrocentra may explain its broad distribution and adaptation to different
 environmental factors, one of which may be pollinator availability. Our
 results highlight the need to describe the reproductive ecology of
 populations instead of species because reproductive systems and ecological
 factors such as plant-animal interactions may differ across their
 distribution range.
description: <strong>Floral visitors</strong>: We selected 32
 individuals of <em>O. macrocentra</em> to study their floral
 visitors. During five consecutive days within the flowering peak, we
 checked 32 plants from anthesis to flower closure, covering three periods
 of the day, morning (09:00–11:00 h), mid-day (12:00–15:00 h), and late
 afternoon (18:00–20:00 h) (n=279 observations in 22 h).  
 <strong>Nectary measurements</strong>: We measured
 the nectary length, upper and lower diameter, and its depth within the
 flowers as an indicator of nectar production. We collected tree flowers
 from 30 different individuals and fixed them in 70% alcohol (n = 76
 flowers).  
 <strong>Pollen Ovule Ratio</strong>: We collected and
 fixed in 70% alcohol three flowers from each of 30 different individuals
 during different anthesis periods to estimate the number of filaments and
 ovules per flower (n=90 flowers). Each flower was divided into four equal
 parts. The number of filaments and ovules in one randomly chosen section
 was tallied and multiplied by four to estimate the total number of
 filaments and ovules per flower. Pollen grain number per flower was
 estimated from one flower of the same 30 individuals. A single closed
 anther per flower was collected and fixed in 70% alcohol. The pollen
 grains were counted by squashing the anthers in a Petri dish under a
 stereoscopic microscope 10× to 30×. Then, the total pollen per flower was
 estimated by multiplying the pollen counts by the mean number of filaments
 of its corresponding individual plant. 
 <strong>Herkogamy</strong>: We collected 90 flowers
 from 30 different individual plants (three flowers per individual)
 throughout different times of the day and fixed them in 70% ethanol. The
 length of the style of each flower and one filament per flower were
 measured with a digital Vernier caliper to the nearest 0.01 mm. If, by
 means of a paired t-test, the length of both floral structures were
 significantly different, we confirmed herkogamy.  
 <strong>Fruit set, seed set, and seed-to-ovule
 ratio</strong>: Five different controlled pollination treatments
 were conducted on 30 plants during the 2019 reproductive season (May).
 Each plant was treated as a block. Flowers were bagged with tulle fabric
 bags before anthesis and randomly assigned to one of the five pollination
 treatments: 1) autonomous self-pollination, 2) manual self-pollination, 3)
 cross-pollination, 4) supplementary pollen or 5) natural pollination.
 Fruits were collected in September 2019. The fruit set (fruit-to-flower
 ratio), seed set (number of seeds per fruit), and seed-to-ovule ratio
 (based on the mean ovule number per plant) were determined. Differences in
 the fruit set were analyzed using a generalized linear model (GLM) with an
 offset variable. Seed/ovule ratios among pollination treatments were
 analyzed using GLM with binomial error distribution and logit link
 function, whereas the seed set was analyzed with GLM with Poisson error
 distribution and logit link function. 
description: Excel, R