Gabriella Sella

Reciprocal egg trading and brood care in a hermaphroditic polychaete worm

Abstract Ophryotrocha diadema is a simultaneous hermaphrodite polychaete worm with a brief protandrous phase. Its mating behaviour was investigated in order to elucidate the relationships between mating system and reproductive biology. A genetically determined yellow or white coloration of the eggs and body walls made it possible to distinguish the egg releaser from the fertilizer. The following main features of the mating system were established. (1) Pairs are formed preferentially between simultaneous hermaphrodites, one partner releasing eggs and the other fertilizing them. There is no selfing. (2) The partners attain spawning synchronization by means of close mutual contact during a fairly time-consuming courtship. (3) Partners regularly alternate sex roles, usually with the same partner more than once in succession. (4) Both partners care for their eggs and protect cocoons of neglected eggs spawned by other pairs.

Sex economy in benthic polychaetes

Among polychaetes sequential hermaphrodites are generally protandrous and their sex allocation strategy appears to fit well with the Ghiselin size advantage hypothesis. In Ophryotrocha puerilis male reproductive success drops with increasing size because females prefer to mate with small males to avoid a costly conflict over sex. Moreover both partners of a pair can simultaneously change sex several times after some spawnings. Females being soon drained of resources, due to the higher reproductive and social costs of their role, switch to the less costly male role and males thus become more apt to function as females. Also in the simultaneously hermaphroditic species so far studied, O. diadema and O. gracilis, the male profit-effort function for reproductive success shows a decreasing slope. Some 80% of the physiological reproductive effort is allocated to the female functions. Simultaneous hermaphroditism is stabilized by slow sperm production and regular egg trading between partners of a pair. Local mat...

Sex Allocation in the Simultaneously Hermaphroditic Polychaete Worm Ophryotrocha Diadema

O. diadema is a simultaneously hermaphroditic polychaete worm with a brief protandrous phase. Pairs are formed preferentially between two simultaneous hermaphrodites, which reciprocally change sex roles and trade eggs. Allocation of reproductive effort during simultaneous hermaphroditic and protandrous phases was evaluated quantitatively, by microscopic examination of both fixed and living specimens. Ovarian tissue is four times as abundant as testicular tissue. The mean number of sperm counted in sectioned animals was taken as an approximation of the number of sperm offered per egg. As expected in species where the mating group always consists of two individuals, this number is low (50 sperm are offered per egg). Like eggs, sperm released at fertilization are replaced within 3 d. Such reduced allocation to testicular tissue allows saving in the physiological cost of sperm production. Efficiency of fertilization in the protandrous phase is low but positively correlated with body length (measured by the nu...

Outcrossing hermaphroditic polychaete worms adjust their sex allocation to social conditions

Sex allocation theory predicts that simultaneous hermaphrodites shift sex allocation facultatively in response to variation in local group size. This study was performed to evaluate the relative investment in each sex function by the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema and to test whether allocation to each sex depends on the number of reproductive competitors. Four experimental groups were set up (in a 2 × 2 factorial design) with small or large group size and with small or large enclosures to control for potential confounding effects of density. We measured the proportion of female and male investment in focal individuals. Results revealed that individuals regulated their reproductive output so that when reproductive competitors were present, the number of female gametes was strongly reduced and the male function increased. In contrast, under monogamy, individuals in small groups produced lower numbers of sperm but had a higher egg output than worms in large groups. Density did not affect sex allocation in our experiment. Our findings provide qualitative support for Local Mate Competition theory, but also show that the pattern of sex allocation specific to this species is more complex than expected by current theory.

Life history and sex allocation in the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema: the role of sperm competition.

Sex allocation theory predicts that, in hermaphroditic organisms, individuals allocate a fixed amount of resources divided among male and female functions to reproduction and that the proportion devoted to each sex depends on the mating group size. As the mating group size increases, hermaphrodites are predicted to allocate proportionally more resources to the male and less resources to the female function (approaching equal allocation to both sexes) to face increased sperm competition. Up to now little experimental evidence has been provided to support the theory in hermaphroditic animals. Facultative shift between male and female allocation in response to variation in local group size does occur in several taxa but not always in the expected direction and not with similar patterns. In the protandric and then simultaneously hermaphroditic polychaete worm Ophryotrocha diadema reproductive resources are flexibly allocated in the protandrous and the hermaphroditic phase. The cost of male reproduction during adolescence is spread over the whole energy budget of the animal as shown by the shortening of lifespan and the lowering of growth rate in individuals with enhanced male expenditure during the protandrous phase. Moreover, in this species, short term sex allocation adjustments differ from those described in other taxa. Individuals regulate their reproductive output so that where reproductive competitors are present, the number of female gametes is strongly reduced but the number of male gametes (although it changes) is not significantly increased. Resources subtracted from the female function are not directly allocated to sperm production, but to expensive male behaviors that are likely to enhance male reproductive success. These results are discussed in the light of the relevance of sexual selection in large populations of hermaphrodites.

CYTOGENETIC CHARACTERISTICS OF A POPULATION OF CHIRONOMUS RIPARIUS MEIGEN 1804 (DIPTERA, CHIRONOMIDAE) FROM A POLLUTED PO RIVER STATION

A population of Chironomus riparius from a Po river station near Moncalieri (a trace-metal polluted station) was studied. In this population was established a great variability of band structure of polytene chromosomes as well as paracentric heterozygous inversions, deletions, deficiencies, partial breaks, diploid chromosome fragments, and changes in functional activity and appearance of heterochromatin. In arms A through F, some bands had an increased size compared to the standard chromosomic map. Some bands appeared in a heterozygous or normal homozygous state or were amplified. In all arms, many condensed stable bands appeared in the decondensed state when compared to the standard map. Asynaptic zones in arms E and G as well as heterozygous Balbiani rings and NORs were established. Very often the 4th chromosome was almost completely heteropycnotic and looded like a ‘pompon’ chromosome. For the first time in this species, a high frequency of ectopic pairings of different arms was observed. Telomeric regions involved in ectopic pairings had a granular appearance, as did some centromeres. The hypothesis is advanced that such a high frequency of structural rearrangements could be correlated with genomic distribution of specific mobile elements.

Sexual conflict and mating systems in the dorvilleid genus Ophryotrocha and the dinophilid genus Dinophilus

Predictions of mating system and sexual conflict theory are applied to mating systems of the best studied among the simultaneously hermaphroditic, sequentially hermaphroditic and gonochoric species of the dorvilleid genus Ophryotrocha and of two species of dinophilids. In the hermaphroditic Ophryotrocha species, the mating system is characterized by pair mating, absence of sperm competition and exchange of sexual roles between partners (egg trading). Features that stabilize the pair bond are analyzed (i.e. breeding sex-ratio, biparental care, mechanisms against cheating). Male-male competition and female preference for small males differentiate the mating system of a sequentially hermaphroditic species from that of a gonochoric species, where females prefer large males as mates. In the gonochoric species of Dinophilus different population structures (either panmictic or with sib-mating) select either for absence of sexual dimorphism and a 1:1 sex ratio or for strong sexual dimorphism and female biased sex ratio.

Structural-functional rearrangements in chromosome G in Chironomus riparius (Diptera, Chironomidae) collected from a heavy metal-polluted area near Turin, Italy

A natural population of Chironomus riparius from the heavy metal-polluted Piedmont station (Italy) was analysed. Chromosome G, on which the Balbiani rings BRa, BRb and BRc are located, was very sensitive to the pollution. The BR system was considered to be an interesting model for studying the response of the genome to the heavy metal pollution. Together, with a standard activity of BRs (BRc>BRb), we observed a clear reversed level of activity of BRb and BRc: BRc was repressed while BRb became activated. Also, there were a number of cells in which BRb and BRc were collapsed. A change in the activity of the nucleolar organizing region (N) was found, from a very high activity till completely collapsed. For the first time it was established that there was an activation at the telomere region of chromosome G. Additional active sites in sections Dc and E2de were observed. The conjugation of both homologues was often disturbed. A high frequency of ectopic pairings was also observed. In addition, structural rearrangements as inversions and deletions were found. Some deletions led to the formation of pompon-like chromosome G. Chromosome G of C. riparius is suggested to be used to test for the presence of genotoxic concentrations of polluting agents in an aquatic ecosystem.

A measure of sexual selection in hermaphroditic animals: parentage skew and the opportunity for selection.

The role of sexual selection in shaping the mating system of hermaphrodites is currently widely accepted. However, a quantification of the intensity of sexual selection in hermaphroditic animals has never been accomplished. We evaluated the opportunity for sexual selection for both the female and the male functions in the simultaneous outcrossing hermaphrodite Ophryotrocha diadema by measuring focal hermaphrodites’ paternal and maternal offspring in experimental replicated monogamous and promiscuous populations, using genetic markers to estimate paternity. Opportunity for sexual selection for each of the two sexual functions was quantified by means of the Crow’s index, i.e. the ratio of variance in progeny number to the squared mean number of progeny. In addition, the extent to which the reproductive success was shared among competing individuals was estimated by means of the Nonacs’s B index. We documented that the strength of selection on the male and female function in hermaphrodites with external fertilization depends on the reproductive context. Under a promiscuous regime, hermaphrodites have higher opportunities for selection for both the male and the female function than under the monogamous regime. Moreover, the reproductive skew for the female function becomes greater than that for the male function, moving from monogamy to promiscuity. In our model system, allocation to one sexual function is opposed by any degree of allocation to the other, indicating that sex‐specific patterns of selection operate in this model species.

Genotoxic effects of chromium onpolytene chromosomes of Chironomus riparius Meigen 1804 (Diptera, Chironomidae)

Abstract Genotoxic effect as tested of chronic exposure to three different concentrations of Chromium (III) on polytene chromosomes of larvae of Chironomus riparius (syn. Chironomus thummi) from the embryonic stage to the IV larval instar for two successive generations. In chromosomes AB, CD, EF and G significant differences of chromosome aberrations were found between exposed and control larvae as well as changes in functional activity (induction of novel puffs not corresponding to those induced during normal larval development in arms A, B, C and E, telomeric and centromeric decondensations especially at telomeres of chromosome G and arm C.). No significant differences were found between the effects of the three treatments nor between the two generations. In chromosome G the Balbiani Ring system (where some permanently active regions are involved in transcription of salivary proteins) appeared as a model for studying the response of the genome to Cr (III) treatment. In approximately one-third of the cells of the exposed larvae, the activity of the Balbiani rings BRc and BRb was reversed. In 10% of the cells of both generations of treated larvae, deletion or collapse of BRc was observed. Pomponlike G chromosomes were present in 6% of the cells. This type of chromoosme appeared either in a very decondesed or in a highly condensed state. In 5 % of the treated larvae the apical region of the chromosome G folded back so that the nucleolar organizer region appeared as if it was at the end of the chromosome. These structural and functional chromosomal changes are interpreted as a reaction of the genome to stressful rearing conditions.