Estudo biossistematicos em tres especies de Bulbophyllum Thouars (Orchidaceae) ocorrentes nos campos rupestres brasileiros

Publication Type:Thesis
Year of Publication:1997
Authors:E. L. Borba
Keywords:Brazil, Bulbophyllum, pollination
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Abstract: The floral biology and phenology of the orchids Bulbophyllum weddellii
(Lindl.) Rchb. f. and B. involutum Borba, Semir & F.Barros
(unpubl.), in Serra do Cipó, Santana do Riacho, and B. ipanemense
Hoehne, in Serra de Camargos, Nazareno, both located in Minas Gerais
state, Southeastern Brazil, were studied. The three species are
pollinated exclusively by females of Pholeomyia (Diptera: Milichiidae)
with a high degree of specificity, probably attracted by oviposition
instincts. In ali species, a successful pollination requires winds with a
speed of 1.5 to 2.0m/s to shake the labellum until the insect is glued
to the column. This mechanism is known in the genus only for these
species, and seems to have evolved in open areas such as the "campos
rupestres" where they occur. In spite of the high rate of pollinia
removal, fruit set is very low (3 to 8%) due to the lack of constancy of
the flies. High levels of herbivory and infrequent flowering also
contribute to the low reproductive success of these species. This
stated, reduced fructification could be interpreted as adaptive,
granting survival and future reproduction in their resource-poor
environment. In the few cases where sexual reproduction occurs, cross
pollination is practically granted through specific mechanisms.
Immediately after its removal from the anther, the pollinarium of the
closely related species B. involutum and B. ipanemense has approximately
double the width of the stigmatic cavity entrance. The pollinarium
takes about 105 to 135 minutes to reduce in size enough for pollination
to occur, and only the width is reduced significantly. Because the flies
stay for up to 12 minutes in the flower after removing the pollinia,
this previously unknown mechanism in Orchidaceae is very important in
preventing self-pollination. In B. weddellii this mechanism does not
occur, and the pollinator does not remain in the fIower after pollinia
removal. The smaller diameter of the stigmatic cavity in B. involutum
reduces in 50% the chances of interspecific pollination with B.
weddellii, so that crossing is unidirectional. This is important to help
ensure the isolation between these two species, which are sympatric,
bloom at the same time and share the same pollinators. Experiments of
self-pollination, intra and interspecific cross-pollination were done
for the three species. Ali of them proved to be self-compatible, and the
occurrence of pollinator is necessary for fruit development. Ali
crossings are caracterized by a high rate of fruit abortion and
development of seeds without embryo, and they are highest in the
interspecific crossings involving B. weddellii. Crossings between the
related species B. involutum and B. ipanemense result in values of fruit
set and seed viability similar to intraspecific crossings. These
results agree with current hypotheses on interspecific crossings
potential reflecting the phylogeny of the species involved. Examination
of pollen tubes and aborted fruits show that a series of factors such as
low pollen germination, incompatibility reactions, and embryo abortion
because of lethal recessive alleles may be the cause of the great number
of fruit abortions and seeds lacking embryos. The low fertility of
crossings between B. weddellii and B. involutum is important to maintain
the isolation of these species. The reproductive biology data provide
support for the separation of B. involutum as a good species, which is
described here for the first time. Comparisons with the closely related
species B. warmingianum Cogn. and B. ipanemense are made. A natural
hybrid between B. weddellii and B. involutum occurring in Serra do Cipó,
B. xcipoense Borba & Semir, is described and its reproductive
biology studied. A system of filters, in which a combination of weak
barriers between these two species result in a strong isolation,
contributes to the extreme rarity of the hybrid and consenquently to the
status maintenance of the parental species. The hybrid attracts the
same pollinators as the parents and is located within the B. weddellii
population, which may cause introgression. However, the hybrid has a
high levei of sterility or self-incompatibility, and labellum
characteristics make pollination inefficient. Due to the location of the
hybrid and the unidirectional crossing between the parents it is
assumed that this hybrid has originated from pollen donation by B.
involutum. The possible consequences of this hybridization event for the
parental species and in the establishment of a new species are

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