Th. Fleming et al., REPRODUCTIVE-BIOLOGY AND RELATIVE MALE AND FEMALE FITNESS IN A TRIOECIOUS CACTUS, PACHYCEREUS-PRINGLEI (CACTACEAE), American journal of botany, 81(7), 1994, pp. 858-867
We describe the breeding system of an autotetraploid trioecious cactus
, Pachycereus pringlei, provide estimates of the fitnesses of males an
d females relative to that of hermaphrodites, and discuss the role pla
yed by pollinators in the maintenance of three sexual morphs. Relative
ly high frequencies of females (45%) and males (26%) exist in coastal
desert populations around Bahia Kino, Sonora, Mexico. They differ from
hermaphrodites in flower size (females only), initiation of the flowe
ring season, number of flowers produced per night and per season, sucr
ose content of nectar, and, in females, number of fruits produced per
season under open pollination and in response to hand-pollination. Maj
or similarities between the sex classes include overall plant size, ne
ctar volume per flower, percent fruit set in open-pollinated flowers o
f females and hermaphrodites, seed mass and number of seeds per fruit,
and pollen mass per flower in males and hermaphrodites. Hermaphrodite
s are self-compatible, and the selfing rate is high (65%). Levels of i
nbreeding depression in selfed fruits and seeds appear to be low. Frui
t set is strongly pollinator-dependent in females but much less so in
hermaphrodites. Relative fitness of males and females, as measured by
annual production of pollen or seeds, is at least 1.5 times higher tha
n that of the corresponding sex function in hermaphrodites. Given the
high selfing rate and apparent lack of inbreeding depression, these fi
tness differences are insufficient to explain the occurrence of trioec
y in this species.