


Estimate of the longevity in experimental
populations established with hybrids of Drosophila prosaltans and D. saltans.
Freschi, Ana Paula P., Marlene K.H. Kobayashi,
Antonio J. Manzato, and Wlademir J. Tadei. Departamento de Biologia – IBILCE,
Universidade Estadual Paulista – UNESP, Rua Cristuvao Colombo, 2265,
Sao Jose do Rio Preto, SP 15054000, Brazil.
Studies of the reproductive isolation of the sibling species of Drosophila
prosaltans and D. saltans show that they cross with each other in the laboratory,
depending on the geographical origin of the flies used. Tadei (1974, 1977)
working with these two species, established 4 groups of experimental populations,
and among these was the group H constituted by populations founded with hybrids
F1 resulting from the crossing
between females of D. prosaltans with males of D. saltans. The populations H_{2},
H_{3} and H_{4} (group H) had been kept by the serial transfer
technique (BuzzattiTraverso, 1955; cf., Tadei and Mourão, 1981) in a constant
temperature room (25 ± 1.0ºC), with censuses
carried out every 7 days, counting themselves for sex and adults and young
being weighed separately. Since the foundation until the aforementioned date
of the publication, 170 full weekly censuses had been carried out. Since then
these populations had been kept only with weekly transfers until May 1999,
when we restarted the full weekly censuses, totaling 100 censuses in this
analysis. With the resumption of the full censuses, it was possible to study
the population parameters related to the size (regarding the number of flies
and biomass), birth rate and mortality of H_{2}, H_{3} and
H_{4}, estimating directly in the populations the longevity of the
flies in conditions of intense competition.
The estimate of the average longevity of the flies was made through
Levene's Method (appendix in Dobzhansky and Pavlovsky, 1961). This method was developed for determing
the longevity in populations where the processes of birth and death are considered
continuous stationary stochastic processes. In this case the gross average
longevity ( ) is given by the number
of surviving individuals divided by the average number of individuals born
in an interval of unitary time. If time's interval for the computation of
births is very long, the correction of the estimate of the average longevity
of the flies becomes necessary. The model of Levene was developed for application
in populations with two weekly transfers. The application of this model modified
for populations with a weekly census is hereby proposed. By accepting the
premises of the model, the expression for obtaining the value is
given by:
By
means of the expression above, the value
(corrected
average longevity) in terms of
is
given by:
In this study the values of
the gross average longevity ( ) and corrected ( ) in the unit of time
of the censuses (weeks) were estimated as well as the values in days for the
corrected average longevity ( *), minimum and maximum
longevity of males, females and total of flies populations H_{2}, H_{3} and H_{4}.
Differences in the longevity of the flies of these populations were verified,
the average longevity being larger in H_{4} (19.9 days), intermediate
in H_{2} (15.0 days) and smaller in the population H_{3} (12.7
days). H_{4} population, in relation to H_{3}, presents an
increase in the average longevity of 38% for males, 33% for females and 36%
for the total of flies. The biggest differences in longevity values are observed
when the upper values of the distribution are considered; H_{4} flies
survive up to 66 days, more than twice the time of survival of H_{2}
and H_{3} flies.
The average values obtained
for birth and mortality rates are equivalent in each population, indicating
a balance of stable size as required by Levene's Model. However, when each
of the taxes is separately considered in the comparison among the three populations,
the differences are significant. These populations show different adaptive
strategies, for, in spite of population H_{4} presenting the smallest
values for birth and mortality rates, its population size is intermediate
in relation to H_{2} and H_{3}, being basically kept because
of the flies’ largest longevity. Due to the fact that these populations
present only small variations in size and in birth and mortality rates along
the time, it was possible to use Levene’s Model, that proved itself
efficient in the estimate of the flies’ longevity. Despite Tadei and
Bicudo (1981) chromosomal analyses having shown that H_{4} population
reconstituted the full chromosomic set of D. saltans eliminating
of D. prosaltans, preliminary results now obtained from our molecular
analyses with markers of AFLP (Amplified Fragment Length Polymorphism) show
that this population still maintains DNA segments of D. prosaltans.
References: BuzzatiTraverso, A.A., 1955, Heredity 9: 153186; Dobzhansky, Th., and
O. Pavlovsky 1961, Heredity 16: 169179; Tadei,
W.P., 1974, Thesis (Master in Genetics), São Paulo: Bioscience Institute,
University of São Paulo, Brazil, USP; Tadei, W.P., 1977, Thesis (Doctor
in Science/Genetics), São Paulo: Bioscience Institute, University of
São Paulo, Brazil, USP; Tadei, W.P., and H.E.M.C. Bicudo 1981, Brazil.
J. Genetics, 4 (4): 549570; Tadei, W.J., and C.A. Mourão 1981, Brazil.
J. Genetics, 4 (2): 149164.