Temperature induces gonadal maturation and affects electrophysiological sexual maturity indicators in Brachyhypopomus pinnicaudatus from a temperate climate
1 Depto de Neurofisiología, Instituto de Investigaciones
Biológicas Clemente Estable, Unidad Asociada de Facultad de
CienciasUniversidad de la República, Avda, Italia 3318,
Montevideo, Uruguay
2 Depto de Fisiología, Facultad de Ciencias-Universidad de la
República, Montevideo, Uruguay
3 Sección de Biología Celular, Facultad de
Ciencias-Universidad de la República, Montevideo, Uruguay
* Author for correspondence (e-mail: lauraq{at}iibce.edu.uy)
Accepted 24 February 2004
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Summary |
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Key words: breeding, temperature sensitivity, electric fish, Brachyhypopomus pinnicaudatus, EOD
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Introduction |
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Communication becomes essential during the breeding season, when fish need
to identify sexually mature, receptive partners of the same species. Reliable
and detectable modulations in EODs could serve as signals of reproductive
state (Kelly, 1983). In
Brachyhypopomus pinnicaudatus the EOD is biphasic (in a head-to-tail
recording), composed of an early head-positive wave (P1) and a late
head-negative wave (P2) (Hopkins,
1991
). During the breeding season, in mature males, the EOD
increases its duration via a longer P2 phase
(Hopkins et al., 1990
;
Hopkins, 1991
;
Caputi et al., 1998
;
Silva et al., 1999
). This
reversible effect upon the EOD has been shown in a related species,
Hypopomus occidentalis, to depend on androgen levels acting directly
on the EO (Hagedorn and Carr,
1985
). B. pinnicaudatus also exhibits morphological
dimorphism during the breeding season
(Hopkins et al., 1990
;
Hopkins, 1991
;
Caputi et al., 1998
; Silva et
al., 1999
,
2003
). In mature males the
tail filament becomes noticeably wider whereas females display protruding
ovaries through their translucent skin.
The geographical distribution of Gymnotiformes is broad: from the Chiapas
Province in Mexico in the north (Vilano
and Balderas, 1987) to the Río de la Plata system in the
south (Mago-Leccia, 1994
).
Over this large geographical area, different environmental cues, which change
seasonally and trigger the onset of breeding in fisheshenceforth
referred to as `zeitgebers'have been identified (Hopkins,
1974a
,b
;
Schwassmann, 1976
;
Provenzano, 1984
;
Hagedorn, 1988
; Silva et al.,
1999
,
2002
,
2003
). Previous studies have
been conducted in the tropical region where, as with many other Neotropical
fishes, most Gymnotiformes breed during the rainy season (Hopkins,
1974a
,b
;
Kirschbaum, 1979
;
1995b
). Nevertheless,
differences in spawning cues have been revealed even within the same species
(Schwassmann, 1976
; Hagedorn,
1986
,
1988
).
Reports on gonadal histology of Gymnotiformes, as well as its annual
changes, are scarce. Moreover, the few existing studies have been done in
Gymnotiformes from the tropical zone. Features of germ cells were briefly
described for Gymnotus carapo (Barbieri and Barbieri
1984,
1985
) and Stenopygus
macrurus (Zakon et al.,
1991
). In Gymnotiformes of the tropical region, gonadal cycles
have been temporally correlated to the alternation of the rainy and dry
seasons (Provenzano, 1984
;
Zakon et al., 1991
).
In temperate and subtropical regions, photoperiod, temperature and their
interaction are demonstrated zeitgebers of reproductive cycles
(Lam, 1983;
Prosser and Heath, 1991
). In
Uruguay (3035°S), the southern boundary of gymnotiform distribution
in America, both water temperature and photoperiod change throughout the year,
whereas rainfall does not show seasonal changes (Silva et al.,
1999
,
2002
,
2003
). We selected temperature
and have analyzed it as the main zeitgeber in the reproduction of B.
pinnicaudatus at this latitude (Silva et al.,
1999
,
2002
,
2003
).
Temperature variations within the natural range for the temperate climate
have a short-term effect upon EOD waveform in B. pinnicaudatus
(Caputi et al., 1998; Silva et
al., 1999
,
2002
). The duration of the
B. pinnicaudatus EOD diminishes as temperature increases. In addition
there is a striking decrease in the amplitude of P2 in water at temperatures
higher than 20°C (which reverses once the temperature is returned to
control values); this effect has been called `waveform temperature
sensitivity' (Caputi et al.,
1998
; Silva et al.,
1999
,
2002
). This effect upon EOD
waveform is so important that at 30°C the EOD is almost monophasic. We
observed this high temperature sensitivity in juveniles and in
non-differentiated adult fish (fish that do not present sexual dimorphism),
whereas fish collected in the breeding season displayed very low temperature
sensitivity (in water at 30°C, changes in amplitude were transient and
returned to control values in approximately 10 min)
(Caputi et al., 1998
; Silva et
al., 1999
,
2002
). Similar observations of
this phenomenon in Gymnotus carapo have been reported
(Ardanaz et al., 2001
) and were
used to prove its peripheral origin.
The aims of this study were: (i) to analyze the histological features of gonads and their annual changes in wild B. pinnicaudatus in relation to water temperature and conductivity; (ii) to analyze annual changes of electrophysiological features linked to the occurrence of the breeding season (EOD waveform and temperature sensitivity), (iii) to evaluate the effect of high temperature acclimation upon gonad histology and EOD waveform.
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Materials and methods |
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Sampling
Fish Brachyhypopomus pinnicaudatus (Hopkins) were captured by
taking advantage of gymnotiform electrogeneration capability and of their
nocturnal habits. They were collected during daytime, when at rest in their
hiding places, using a `fish detector' (a portable electrode amplifier
loudspeaker assembly that detects the presence of fish from a distance up to 2
m) and a hand net. The fish were collected from a small lake, El Tigre,
(Department of Treinta y Tres, 33°18 'S, 54°35 'W) and
neighboring sites. During 1999 and 2000, samples were taken once a month
(springsummer) or every 2 months (autumnwinter). A total of 89
individuals were used in this study. Temperature and conductivity measurements
were taken at midday (digital thermometer TM-915; Lutron, Montevideo, Uruguay;
TDSu Testr 3; Cole Parmer, Chicago, USA, respectively). B.
pinnicaudatus were identified using morphological and
electrophysiological cues (Hopkins,
1991). Adults measured 1218 cm. Adults collected in the
non-breeding season did not present any characteristics of sexual dimorphism,
and are referred to as `non-differentiated fish'.
Histology
Animals were subjected to hypothermia and killed by decapitation. Gonads
were removed immediately afterwards and were fixed in Bouin's solution,
dehydrated in ethanol and embedded in paraffin wax. Sections (57 µm
thick) were stained with Hematoxylin and Eosin and mounted in Entellan (Merck,
Darmstadt, Germany) (Ganter and Jolles,
1970). Sections were examined and photographed under an
OlympusVanox light microscope (Kodak Gold, 100 ASA, 35 mm film).
Measurements of germ cell size were carried out directly under the microscope
using an ocular micrometer (1/100 mm; E. Leitz, Wetzlar, Germany). An average
of 35 cells per stage were measured. To evaluate if the different cell types
were present throughout the entire gonad, 3 ovaries and 4 testes of fish
captured in October were entirely sectioned from rostral to caudal end. In
both cases it was clear that cell types were not polarized and were present
along the entire axis. We therefore worked with sections of the mid gonad. A
total of 70 individuals (40 males and 30 females) were used in this
analysis.
To quantitatively compare the distribution of different cell types throughout the annual cycle, three months were selected: July, October (non-breeding season) and December (breeding season). We analyzed three different sections of each fish, and four fish were used in each group. Germ cells were observed and classified according to their sizes and main morphological characteristics. Quantification of male germ cells was done using a WhippleHauser grid with a total of a hundred squares. Each square was counted as having the cell of the predominating cyst, and the results were expressed as percentage of total cells of the section. Quantification of female germ cells was done by measuring the area each cell type covered, and the results were expressed as percentage of total area of the section (relative area). All measurements were done using computer software (Image Tool, San Antonio, USA).
EOD recordings
Head-to-tail EOD recordings were obtained as described by Caputi et al.
(1998) and Silva et al.
(1999
) throughout the
experiments. Fish were placed in a perforated plastic cylinder (3 cm diameter,
length adapted to the fish's length) with two Nichrome electrodes (at opposite
ends) connected to a high input impedance (G
) buffer amplifier. The
cylinder was immersed in a 30 cmx20 cmx15 cm tank. The outputs of
the amplifier were connected to a digital oscilloscope. Averages of 64 EODs
were stored in a PC for further analysis with software developed in the
laboratory. This recording device restricted fish movements that could
interfere with EOD recordings. Water temperature was changed following
different protocols (see below) and recorded from inside the cylinder. In
order to favor comparisons, EODs were normalized in amplitude with respect to
P1. No voltage calibration will therefore be included. P1 and P2 durations
were measured at 10% of peak amplitude. The lengthening of P2 duration typical
of the mature breeding male was chosen as an indicator of electrophysiological
dimorphism. In order to avoid the influence of temperature on EOD duration, we
quantified electrophysiological dimorphism by the duration of P2/duration of
P1 (DP2/DP1) ratio.
Temperature effects on EOD waveform
The experiments carried out to evaluate the temperature sensitivity of EOD
waveform are detailed elsewhere (Caputi et
al., 1998; Silva et al.,
1999
; Ardanaz et al.,
2001
). Briefly, fish were placed in 20°C water for 1 h, after
which their EOD was recorded and kept as control value. The recording cylinder
was then gently lifted, drained and submerged in water at 30°C. EOD
recordings were obtained 30 min after the temperature step was imposed. The
peak amplitude of P2/peak amplitude of P1 (AP2/AP1) ratio was used to
represent waveform changes with temperature. A temperature sensitivity
waveform index (TS index) was constructed with the AP2/AP1 measured at
20°C (control) and 30°C (after 30 min), the difference being divided
by the value at 20°C, that is:
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Acclimation experiments
Non-differentiated adults were acclimated at a constant temperature of
28°C for 30 days with a 12 h:12 h L:D photoperiod and constant
conductivity (100 µS cm-1) following the same procedures as
described by Silva et al.
(1999
). Briefly, conductivity
was controlled daily and kept low and constant by addition of distilled water;
fish shared tanks in pairs and were fed every other day. To evaluate changes
in gonadal profile, fish collected in July were divided in two groups: a
control group, which was recorded before histological examination
(N=7: 3 males and 4 females), and a group that was acclimated during
the month of August (N=6: 4 males and 2 females); EOD recordings were
then performed before gonad histological examination. The same protocol was
repeated with a group of fish collected in September (control group
N=6: 4 males and 2 females; acclimated group N=6: 4 males
and 2 females).
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Results |
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The ovigerous lamellae, covered by germinal epithelium, extended from the ovary wall to the center of the organ. Oocytes grew as a cluster within each lamella. Five different germ cell types were recognized inside the lamellae (Table 1). Oogonias were round cells that had scarce cytoplasm, a central nucleus and a single nucleolus. They were observed alone or in small aggregations (Fig. 1A). Previtellogenic oocytes were round or polygonal and presented strongly basophilic cytoplasm. They had a central, spherical nucleus, with chromatin observed in fine granules (Fig. 1B). In larger previtellogenic oocytes, numerous nucleoli close to the nuclear membrane could be seen, as well as lampbrush chromosomes. Oocytes in the lipid yolk stage were present in a wide range of sizes. Lipid droplets started to appear peripherally and, as the oocytes grew, the droplets increased in number and size and invaded the cytoplasm inwardly. The nucleus contained many nucleoli and sometimes appeared deformed by the large surrounding lipid droplets. At this stage the chorion (acellular membrane located between follicle cells and oocyte) started to be visible. In these oocytes the width of the chorion was 5.21±3.39 µm (mean ± S.D.; Fig. 1C). Acidophilic yolk (or protein yolk) oocytes presented yolk granules that started to appear close to the nucleus. The chorion was 8.87±2.40 µm in width and did not differ significantly from chorions of advanced protein yolk oocytes. In advanced stages, the protein yolk had pushed the lipid yolk to the periphery and the chorion was 7.38±2.28 µm (Fig. 1E). In both lipid and protein yolk oocytes, radial stratification could be observed in the chorion. Fully grown oocytes contained a very large amount of protein yolk which paraffin wax did not penetrate entirely. This methodological disadvantage made it very difficult to observe intact fully grown yolk oocytes and did not allow sections of preovulatory oocytes. However, morphological features from remaining parts of fully grown oocytes allowed their identification. Both atretic follicles and post-ovulatory follicles were also observed.
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|
Male gonad histology in B. pinnicaudatus
The testis is a paired organ, of a more-or-less triangular section,
positioned ventrally and extending posteriorly to the dorsal wall of the
abdominal cavity. The gonads joined anteriorly in a common genital sinus,
which connected to a genital papilla located ventrally between the opercula.
Testicles were organized in a lobular testicular structure consisting of
branching tubuli and interstitial tissue among them. The seminiferous tubule
had a central lumen surrounded by the germinal epithelium. This was organized
in cysts, where spermatogenesis occured. Sertoli cells formed the wall of the
cyst. Each cyst contained germ cells in the same stage of development. Four
different cell types were identified inside the cysts
(Table 2). Spermatogonia were
large cells with a very conspicuous and spherical nucleus. It was possible to
see the peripherical condensed chromatin, whereas the cytoplasm was very
scarce and lightly basophilic (Fig.
2A). Spermatocytes were smaller in size and their nucleus
contained granular chromatin in different stages of condensation depending on
the development of the spermatocyte (Fig.
2C). Spermatids, which were in the cysts, were small and the
nucleus was strongly basophilic (Fig.
2C). Late spermatids were observed `oriented' inside the cysts,
with their heads towards the periphery and their tails towards the center
(Fig. 2E). Finally, free sperm
in the lumen were smaller still and showed a strongly basophilic nucleus
(Fig. 2E).
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|
Annual cycle of gonads
Four histological phases were used to describe the ovarian annual cycle:
regressing/resting, recovering, maturing and mature. At all stages of the
cycle previtellogenic oocytes were present. The regressing/resting ovary was
present from February to July; this stage indicated the end of the spawning
season of the species. It was characterized by hemorrhagic connective tissue,
oogonia and small previtellogenic oocytes. The ovary gradually started to
increase the number of previtellogenic oocytes, and in July some ovaries
started the recovering stage (Fig.
1B). The recovering ovary was observed between August and October;
most oocytes were in the process of growth and deposition of lipid yolk
(Fig. 1D). They were in
lamellae with well-defined limits. The maturing ovary was present in October
and November. It contained mainly two kinds of oocytes: with lipid yolk or
protein yolk, the latter in different states of deposition and growth. The
lamellae lacked well-defined limits due to the size of the oocytes. The mature
ovary was found from November to January. It consisted mainly of large fully
grown oocytes while a few others were still in the process of deposition of
protein yolk (Fig. 1F). In
addition, atretic follicles could be observed as well as post-ovulatory
follicles, which indicated spawning. The relative area covered by oocytes
showing different degrees of maturation, varied throughout the year as can be
observed in Fig. 1G
(N=4 for each experimental group).
Four histological phases were used to describe the testicular annual cycle: regressing/resting, recovering, maturing and mature. The regressing/resting testicle was observed from February to July. At the beginning of this period hemorrhagic tissue was present and the germ cells were mostly spermatogonia with very few pools of unspawned sperm. Towards June and July testis showed an almost homogeneous population of large spermatogonia and an occasional cyst of spermatocytes (Fig. 2B). The recovering testis was observed from August to October. It was possible to observe some spermatogonia and cysts of spermatocytes in different stages of development, as well as spermatids (Fig. 2D). The maturing testis was found in October and November. It contained mainly cysts of spermatocytes and spermatids, many of which were oriented inside the cysts. The mature testis was observed from December to February. Spermatids in different stages of development were observed as well as large pools of sperm in the lumen. The germinal epithelium had become thinner (Fig. 2F). The gonadal profile, represented in Fig. 2G as the relative number of different germ cells throughout the year, showed the expected annual cycle (N=4 for each experimental group).
Annual cycle of environmental variables
The breeding season of B. pinnicaudatus has been reported to occur
from November to January (Silva et al.,
2003). We studied the annual cycle of environmental variables in
one same habitat over 2 years. Water conductivity was low and relatively
constant throughout the study period (47±21.62 µS cm-1 in
1999 and 53.75±40.68 µS cm-1 in 2000:
Fig. 3A). Rainfall occurs all
year long (raw data obtained from National Weather Service, Uruguay. Seasonal
data was obtained over the last 30 years; summer: 312±84 mm; autumn:
302±79 mm; winter: 263±89 mm; spring: 307±84 mm) and does
not show significant differences between seasons (ANOVA, P=0.188).
Water temperature measurements, always taken at noon, showed the expected
seasonal changes, ranging from 11°C in June to 34°C in January and
February, as shown in Fig.
3C.
|
Annual changes of electrophysiological characteristics
Electrophysiological dimorphism of the male, measured by the ratio DP2/DP1,
was approximately 1 in the non-breeding season and gradually increased towards
the summer, reaching its peak mean value, 1.52, in December
(Fig. 3D). By contrast, the
temperature sensitivity of the EOD waveform increased towards the winter, and
decreased towards the summer (Fig.
3E). Its lowest mean value, 0.66, coincided with high water
temperatures, marked male electric dimorphism and extreme photoperiod (14 h:10
h L:D). It is clear that gonads reached stages of maturity in coincidence with
high water temperature and extreme photoperiod. Moreover, gonadal maturity
appeared together with electrophysiological dimorphism and a low TS index.
EOD recordings of two fish, a non-differentiated adult and a mature male, subjected to a temperature step from 20° to 30°C are shown in Fig. 4A. At 20°C both fish showed the typical biphasic EOD, whereas after 30 min at 30°C the non-differentiated adult presented a practically monophasic EOD. This fish presented high temperature sensitivity (Fig. 4A, left). However, at 30°C, the mature male presented an EOD similar in amplitude to the control recording, thus displaying low temperature sensitivity (Fig. 4A, right). TS indices of fish with resting/regressing gonads (stage 1 in Fig. 4B,C) were significantly higher than the values presented by fish with mature gonads (stage 4 in Fig. 4B,C) in both males and females (MannWhitney test, P<0.05; n1=4, n2=4).
|
Induction of sexual maturity by acclimation
To evaluate the role that temperature may play in the onset of the breeding
season, acclimation experiments were carried out (28°C, constant and low
water conductivity, 12 h:12 h L:D). The effects of acclimation upon
electrophysiological dimorphism and gonad histology were analyzed.
Non-differentiated males were acclimated to high temperature for 30 days.
In contrast to the effects of sustained low-temperature acclimation, which
induces a decline in morphological and electrophysiological signs of sexual
maturity (Silva et al., 1999;
see Discussion), the EOD waveform after 28°C-acclimation showed a
lengthened P2 phase in comparison to the waveform before the experiment
(Fig. 5A). Males captured in
September and acclimated during 30 days at 28°C were compared to those of
the natural habitat captured before the acclimation experiment
(August/September). A second control group was captured from the natural
habitat after the acclimation experiment was over (October). These two control
groups did not present significant differences in the DP2/DP1 ratio (Figs
3D,
5B). The acclimated males
presented a significant change in EOD waveform in comparison to control males
(Fig. 5B) of August/September
(MannWhitney test, P<0.05, n1=4,
n2=5) and October (MannWhitney test,
P<0.05, n1=4, n2=5).
Acclimated males presented a DP2/DP1 ratio of 1.51±0.17, which
indicated a very lengthened second phase, similar to those of fish captured in
December (breeding season), which was 1.52±0.32
(Fig. 3D).
|
Testes of fish captured in July showed an almost homogeneous population of spermatogonia. Control fish collected in September from the wild presented a gonadal profile that was slightly more advanced than in July, in which it was possible to observe spermatogonia and cysts of early spermatocytes (Fig. 6). Fish acclimated for 30 days at 28°C showed a striking change in the cell profile of the testes in experiments carried out during August (fish collected in July; see `Accl.1' in Fig. 6) and during October (fish collected in September; see `Accl.2' in Fig. 6). The number of spermatogonia decreased, and cysts with spermatocytes in different stages of development appeared in great numbers. In addition, a very significant increase in free spermatozoa was observed, grouped in large pools in the lumen. The overall aspect of the gonads was similar to those of breeding fish captured in November. Females also showed differences between control and experimental groups: ovaries were dominated by previtellogenic oocytes in control groups, whereas acclimated groups showed larger oocytes in different vitellogenic stages (data not shown).
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Discussion |
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It is interesting to note that the gonads of B. pinnicaudatus, as
those of all studied Gymnotiformes
(Kirschbaum, 1995a), are
located ventrally and exit (together with the intestine) at an extremely
rostral position. This design is very rarely seen in teleosts
(Kirschbaum, 1995a
). As with
other Gymnotiformes, this family presents an EO that runs from just behind the
chin (ventral surface) to the tip of the caudal filament
(Bennet, 1971
). In ontogenetic
studies in B. pinnicaudatus it has been observed that the anus and
the anterior margin of the anal fin move rostrally during development
(Franchina, 1997
). These data
suggest that the extension and location of the EO may have influenced a gonad
displacement away from the most common location of teleost gonads.
Temperature as a zeitgeber of reproduction
Different environmental variables (temperature, photoperiod and
conductivity) may modulate gonadal histology. Gymnotiformes have been mainly
studied as tropical fish, and their reproductive cycles have been associated
to the changes in conductivity related to the alternation of rainy and dry
seasons (Kirschbaum, 1979).
This study addresses the analysis of zeitgebers in a gymnotiform in the
temperate region. It is well established that photoperiod and temperature are
two environmental cues used by temperate-zone teleost fishes in the regulation
of their reproductive cycles (Lam,
1983
; Prosser and Heath,
1991
; Kirschbaum,
2000
). As expected in this region, temperature and photoperiod
showed changes throughout the year whereas water conductivity was relatively
low and constant (Silva et al.,
2003
).
In a previous study in a natural population of B. pinnicaudatus we
made a preliminary identification of the breeding season from November to
January, based upon data of population structure, external dimorphic
morphological signs and EOD waveform
(Silva et al., 2003). Our
present results confirm this identification of the breeding season. There was
a gradual maturation of germ cells towards November/December, and a clear
temporal correlation between high water temperature, extreme photoperiod (10
h:14 h D:L) and the appearance of sexual maturity demonstrated by gonadal
characteristics.
Gymnotiformes have been bred in captivity by putting fish in conditions
that closely resemble the natural habitat (Kirschbaum,
1979,
1995a
;
Silva, 2002
). Kirschbaum
(1979
) kept fish in large
tanks, in groups of at least five, and manipulated many variables at once:
conductivity, water level, and acoustic equivalent of rainfall. In Mormyrids
that have been similarly bred in the laboratory (Kirschbaum,
1987
,
1995a
,
2000
), captivity under regular
conditions causes rapid and profound changes in the endocrine system that
result in a dramatic decline of sexual maturity (Landsman,
1991
,
1993
). In this study, fish
were subjected to captivity conditions that did not resemble the natural
breeding habitat: social isolation, small individual tanks, constant
daynight cycle (12 h:12 h), and low and constant water conductivity.
Notably, we successfully induced gonadal recrudescence by manipulating water
temperature alone.
Low temperature acclimation of mature B. pinnicaudatus provokes a
decline in morphological and electrophysiological signs of sexual maturity
(Silva et al., 1999). The
caudal filament becomes thinner in males and their EODs become shorter, losing
all male characteristics after 30 days
(Silva et al., 1999
). In the
present study, non-differentiated fish kept in the high-temperature
acclimation set-up underwent changes in gonad histology: both males and
females were induced to mature. The gonads of fish after acclimation were very
similar to those captured from the wild in November, during the breeding
season; spawning was not achieved in captivity under these conditions.
High water temperature is sufficient to trigger gonadal maturity and therefore confirms its role as a zeitgeber of reproduction of B. pinnicaudatus in the temperate region. In the temperate region there are two main environmental factors that change seasonally (temperature and photoperiod); our results allowed us to conclude that photoperiod is not necessary for the onset of sexual maturity. Nevertheless, its influence upon the reproduction of this species cannot be ruled out.
The precise route by which environmental factors influence the
neuroendocrine system has not been unraveled so far. In cyprinids, temperature
is the main environmental factor: warm temperatures cause an increase of basal
gonadotrophin release and of pituitary responsiveness to the hypothalamic
factor GnRH, which in turn induces gonadal recrudescence (high gonadosomatic
index and testosterone levels) (Hontela
and Peter, 1978; Breton et al.,
1980a
,b
;
Peter, 1981
; Razani et al.,
1988a
,b
;
Lin et al., 1996
).
Electrophysiological sexual maturity indicators
Gonadal maturation obviously implies the secretion of steroid hormones,
which play a major role in mediating reproductive behavior, either by acting
directly on brain structures governing certain behavior patterns or by acting
indirectly to influence behavior through their effects on the development of
secondary sexual characteristics (Liley
and Stacey, 1983). In Gymnotiformes, several studies have
demonstrated that different parts of the electrogenic system are targets of
steroid hormones. For example, the sexual dimorphism of EOD waveform, a
peripheral phenomenon, has been shown to be causally linked to naturally
occurring or experimentally manipulated levels of sex steroid hormones
(Zakon, 1998
;
Zakon et al., 1999
). In
addition, the emission of social signals during breeding behavior, governed by
certain brain nuclei, is also modulated by steroid hormones
(Dulka and Maler, 1994
;
Dulka et al., 1995
;
Dulka and Ebling, 1999
;
Dunlap and Zakon, 1998
;
Dunlap, 2002
). In B.
pinnicaudatus, electric sexual dimorphism and temperature sensitivity are
driven by steroid hormones and have been postulated as sexual maturity
indicators (Silva et al.,
2002
). In this study hormone measurements were not carried out,
but these two maturity indicators were measured in the wild and experimentally
modulated.
Male electrophysiological dimorphism showed an annual cycle and was evident
during the breeding season. This electrophysiological feature coincided, as
expected, with the appearance of gonadal maturity, as well as with the typical
breeding habitat environmental conditions (high water temperature and extreme
photoperiod). This EOD dimorphism was successfully induced in acclimated
males. These fish, placed for 1 month at a high and sustained temperature,
significantly lengthened the P2 phase of their EOD. On the other hand, low
temperature acclimation of sexually mature B. pinnnicaudatus induced
shortening of their characteristically lengthened P2 phase after 15 days
(Silva et al., 1999).
Captivity, regardless of housing conditions, exerts similar effects in
Mormyrids: EOD sexual identity is rapidly lost in relation to a decay of
androgen levels (Landsman,
1991
,
1993
; Landsmann, 1995). We
have, through 28°C-acclimation, achieved effects similar to those of
androgen treatments (Silva et al.,
1999
). Our results reinforce the idea that acclimation affects
hormone levels which account for sexual dimorphism.
EOD temperature sensitivity is probably a widespread phenomenon, and has
been described so far in B. pinnicaudatus and Gymnotus
carapo (Caputi et al.,
1998; Silva et al.,
1999
; Ardanaz et al.,
2001
; Silva et al.,
2002
). EOD temperature sensitivity showed annual changes: it was
high during the non-breeding season and practically null during breeding;
moreover, it was negatively correlated to gonadal stages. Accordingly,
juveniles show very high temperature sensitivity all year round, including
summer (Silva et al., 1999
).
Previous studies have also demonstrated that acclimation and androgen
treatment induce a decrease in temperature sensitivity in both B.
pinnicaudatus and G. carapo (Silva et al.,
1999
,
2002
;
Ardanaz et al., 2001
). In the
same sense, sexually mature B. pinnicaudatus increase their
temperature sensitivity when acclimated at 20°C during 30 days, and
decrease it when later acclimated at a high temperature
(Silva et al., 1999
). The
decrease in EOD temperature sensitivity, observed during the breeding season,
may be interpreted as a way of protecting the communication value of the
second phase of the EOD, thus allowing this lengthened phase to be a reliable
and detectable sign of reproductive state (in otherwise unfavorable
conditions).
This study provides crucial evidence to prove that high water temperature is sufficient to induce gonadal maturity in B. pinnicaudatus, probably acting through the hypothalamushypophysial axis. In addition, temperature also induces male EOD dimorphism and a decrease in temperature sensitivity, two electrical phenomena which emerge as a direct consequence of gonadal maturity.
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References |
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