Gender-role alternation in the simultaneously hermaphroditic freshwater snail Physa acuta: not with the same partner

In simultaneous hermaphrodites, gender conflicts that arise from two potential mates sharing the same gender preference may be solved through conditional reciprocity (or gamete trading). Conditional reciprocity had initially been considered widespread, but recent studies suggest that its real occurrence may have been overestimated, possibly because most mating observations have been performed on isolated pairs of individuals. Some resulting patterns (e. g., non-random alternation of sexual roles) were indeed compatible with conditional reciprocity but could also have stemmed from the two partners independently executing their own mating strategy and being experimentally enforced to do so with the same partner. Non-random alternation of gender roles was recently documented in the simultaneously hermaphroditic freshwater snail Physa acuta. To distinguish between conditional and unconditional gender alternations, we observed copulations of individually marked snails reared at three contrasted densities. We showed that density affected the overall frequency of copulations during the first 2 days of the experiment with high-density boxes showing more copulations than low density boxes, but it did not affect gender alternation patterns. A change in gender role was observed more often than expected by chance over two successive copulations by the same individual, confirming previous studies. However, gender switches did not preferentially occur with the same partner. We conclude that gender alternation is not due to conditional reciprocity in P. acuta. It may rather stem from each individual having a preference for gender alternation. We finally discuss the mechanisms and the potential extent of this unconditional reciprocity.

Correlated evolution of mating strategy and inbreeding depression within and among populations of the hermaphroditic snail Physa acuta

Inbreeding depression is one of the main forces opposing the evolution of self-fertilization. Of central importance is the hypothesis that inbreeding depression and selfing coevolve antagonistically, generating either low selfing rate and high inbreeding depression or vice versa. However, there is limited evidence for this coevolution within species. We investigated this topic in the hermaphroditic snail Physa acuta. In this species, isolated individuals delay the onset of egg laying compared to individuals having access to mates. Longer delays (''waiting times'') indicate more intense selfing avoidance. We measured inbreeding depression and waiting time in a large quantitative-genetic experiment (281 outbred families derived from 26 natural populations). We observed large genetic variance for both traits and a strong positive genetic covariance between them, most of which resided within rather than among populations. It means that, within populations, individuals with higher mutation load avoided selfing more strongly on average. This genetic covariance may result from pleiotropy and/or linkage disequilibrium. Whatever its genetic architecture, the fact it emerges specifically when individuals are deprived of mates suggests it is not fortuitous and rather reflects the action of natural selection. We conclude that a diversity of mating strategies can arise within populations subjected to variation in inbreeding depression.

Indicazioni e risultati della laparostomia retroperitoneale nel trattamento della pancreatite acuta necrotizzante infetta [Indications and results of retroperitoneal laparostomy in the treatment of infected acute necrotizing pancreatitis]

The aim of this study is to describe personal experience with retroperitoneal laparostomy in the management of infected acute necrotizing pancreatitis. The presence of an infected phlegmon requires surgical debridement and drainage. The surgical approach can be either an anterior laparotomy with irrigation and drainage (which can be either an open or closed laparotomy) or a posterior laparostomy. Three patients (2 men and 1 woman) presented with an unfavourable course of their acute necrotizing pancreatitis despite the administration of broad spectrum antibiotics. A posterior laparostomy with necrosectomy and drainage was performed. The postoperative course was slowly favorable in all 3 cases. Abdominal CT is the best modality for the detection and follow-up of pancreatic necrosis. CT-guided fine needle aspiration can detect superinfection of areas of necrosis. Posterior laparostomy presents several advantages compared to an anterior approach. There is no contamination of the peritoneal cavity; the integrity of the abdominal wall is respected. The necrosectomy is equally complete and the drainage is better as it is direct and posterior.

Size-dependent predation by Dugesia lugubris (Turbellaria) on Physa acuta (Gastropoda): experiments and model

1. We investigated experimentally predation by the flatworm Dugesia lugubris on the snail Physa acuta in relation to predator body length and to prey morphology [shell length (SL) and aperture width (AW)]. 2. SL and AW correlate strongly in the field, but display significant and independent variance among populations. In the laboratory, predation by Dugesia resulted in large and significant selection differentials on both SL and AW. Analysis of partial effects suggests that selection on AW was indirect, and mediated through its strong correlation with SL. 3. The probability P(ij) for a snail of size category i (SL) to be preyed upon by a flatworm of size category j was fitted with a Poisson-probability distribution, the mean of which increased linearly with predator size (i). Despite the low number of parameters, the fit was excellent (r2 = 0.96). We offer brief biological interpretations of this relationship with reference to optimal foraging theory. 4. The largest size class of Dugesia (>2 cm) did not prey on snails larger than 7 mm shell length. This size threshold might offer Physa a refuge against flatworm predation and thereby allow coexistence in the field. 5. Our results are further discussed with respect to previous field and laboratory observations on P acuta life-history patterns, in particular its phenotypic variance in adult body size.

Effect of mating history on gender preference in the hermaphroditic snail physa acuta

Several internally fertilizing hermaphroditic animals can only perform one sexual role at a time. In such species, two individuals that engage in a copulation may have different interests in acting as male or female. A gender choice must be made which, if both individuals have the same preference, may give rise to a severe sexual conflict. Here we tested the hypothesis that gender choice could be influenced by mating history, using the freshwater snail, Physa acuta. We recorded the copulatory behaviour of 240 pairs composed of a focal individual and a partner, each either short- or long-isolated. We found that the time to the first copulation was unaffected by isolation status, suggesting that first contacts in this species are random processes. In contrast, the duration of copulations and the frequency of rejection behaviours suggested that individual gender preference switches from male biased to female biased as isolation increases. In addition, snails rejected copulations more frequently when presented to a partner with the same isolation status. Reciprocity, measured as the rate of gender swapping between the first and second copulations, was high irrespective of gender status. We suggest possible evolutionary causes for this gender preference switch and discuss its potential importance in natural population as well as its consequences for the maintenance of hermaphroditism

Les paramètres du cycle vital de Physa acuta (Gastropoda, Mollusca) en milieu expérimental.

The values of the life history parameters expressed in the Lotka's equation were measured in the experimental conditions (20ºC, food ad libitum) for the aquatic pumonate Physa acuta. The estimated fitness value allows the population to double in about 4 weeks. The life cycle is very short (about 3 times shorter than for Lymnaea peregra in similar conditions) because of the important relative size of the eggs, a very high growth rate and an early maturity. This kind of strategy seems adaptive in eutrophic and temporary pools, where the adult mortality is important and density-independant. While the longevity shows very poor correlations with all other parameters, adult size, age at maturity and fecundity are strongly correlated. Structural and functionnal interpetations of these correlations are proposed. A mixed strategy seems a good hypothesis for this usually bivoltine species: the little-size, early-maturity and high-fecondity strategy may be selected during the summer, and the big-size, delayed-maturity and poor fecundity strategy during the winter

Experimental evidence of inbreeding avoidance in the hermaphroditic snail Physa acuta

Inbreeding depression should select for inbreeding avoidance behaviours. Here we test this hypothesis in two populations of the simultaneous hermaphroditic freshwater snail Physa acuta. We recorded the copulatory behaviour of 288 pairs of sib-mates, non-kin mates from the same population, or non-kin mates from two different populations. We find that kin discriminatory behaviours exist in this species, exclusively expressed by individuals playing the female role. We discuss the relevance of our finding in the context of the evolution of recognition systems and the consequences of such a behaviour in natural populations.

A comparison of the energy budgets of two freshwater Pulmonates: Lymnaea peregra (Müller) and Physa acuta (Drap.)

The energy budgets of two freshwater gastropds, Lymnae peregra and Physa acuta, were compared in similar experimental conditions (20ºC, fed ad libitum with 24h-decayed lettuce), and found to differ in several ways. 1) L. Peregra has a higher assimilation efficiency than P. acuta (72% vs 60%). 2) These species assimilate different components of the ingested food: P. acuta uses a smaller, but more energetic part (probably mainly bacteria), whereas L. peregra assimilate a larger, but less energetic part (probably mainly cellulose). 3) L. peregra allocates more of its assimilated energy to oxygene consumption and mucus production (maintenance investments), wheras P. acuta invest more in growth and reproduction (production investments). Such differences are relevant to the natural habitat of these two species: P. acuta colonizes warm, eutrophic and temporary pools, where decaying material constitue the main part of available resources, and where adult mortality is high and impredictible. By contrast, L. peregra is frequently found in colder, oligotrophic and predictible environements, where living primary producers constitute the main part of available resources, and where biotic interactions are important factors of mortality.

L' "altro sé" : rappresentazioni letterarie d'opposizione nel Novecento italiano

Nel secolo scorso è emersa in Italia un'originale modalité di rappresentazione letteraria che trasfigura i caratteri dell'identità per costruire una particolare forma di alterità, un "altro sé". Infatti, a causa di uno stato di crisi identitaria dello scrittore, in Silo ne e in Carlo Levi si assiste alla costruzione di figure di alterità (il "cafone", i contadini del Sud), rappresentazioni letterarie di quei soggetti socialmente subalterni che questi scrittori volevano riscattare ed emancipare. La tematizzazione dell'autorappresentazione che si riscontra in Brancati e in Pasolini va invece in un'altra direzione, diventa il progetto di una scoperta: la formalizzazione retorica di un'identità "altra" all'interno della propria individualità, letterariamente oggettivata e formalizzata. Brancati, cosi riletto e rivalutato, mostra la crisi dell'autorappresentazione, cioè del posizionamento d'autore nel momento del passaggio dal fascismo all'antifascismo, e quindi il processo di costruzione di un'identità "altra", più forte, che renda possibile la scrittura. Il brancatismo, cioè quel fenomeno culturale legato alia fortuna della narrativa brancatiana, ha anestetizzato questa dimensione critica della scrittura brancatiana riconducendo le sue figure e le sue trame narrative all'interno del discorso dell'alterità, iunzionale a una normalizzazione e borghesizzazione della società italiana del dopoguerra.La tesi propone quindi una teoria della rappresentazione italiana nel Novecento che, prendendo le mosse dalla critica postcoloniale, individua un'originale modalità di rappresentazione per cui gli elementi di "identità" vengono letterariamente trasfigurati in un'"alterità". Individuata questa esperienza, spiccano con inedita intensità le proposte di Carlo Levi per una nuova Italia, che incorporasse il sostrato "primitivo" nell'Italia contemporanea e futura; la progettualità delle esperienze artistiche di Pasolini dagli anni '50 alia sua scomparsa; e il percorso di Sciascia dall'acuta e innovativa riflessione sul pirandellismo degli esordi al ripiegamento sulla "sicilitudine" degli anni '80.