Interference of asexual and sexual reproduction in the green hydra

The green hydra, Hydra viridissima, has three sexes: hermaphrodite, male, and female. I investigated the reproductive strategies of the green hydra and the relationship between asexual budding and sexual reproduction. The proportion of mature individuals in the asexually reproducing population increased with increasing temperature. Sexual reproduction did not interrupt asexual budding in hermaphrodites or males; sexual reproduction did interrupt asexual budding in females. Sexual reproduction also resulted in exponential population growth during the reproductive season. The number of asexual buds on each parental individual was positively correlated with the parental individual size in asexual individuals and in males. The same positive correlation was found between the number of testicles and the size of males. These correlations reflect a common tendency in asexual and sexual reproduction: larger parental individuals have a greater number of propagules or gametes. No correlation was found between size and buds or size and gonads in hermaphrodites; hermaphrodites had at most one asexual bud and were significantly larger than males, females, and asexual individuals. The larger size of hermaphrodites supports the hypothesis that producing both female and male gonads is more energetically costly than producing only one type of gamete (gonochorism).     

Sex‐specific reproductive investment of summer spawners of Illex argentinus in the southwest Atlantic

Energy investment in reproduction and somatic growth was investigated for summer spawners of the Argentinean shortfin squid Illex argentinus in the southwest Atlantic Ocean. Sampled squids were examined for morphometry and intensity of feeding behavior associated with reproductive maturation. Residuals generated from length‐weight relationships were analyzed to determine patterns of energy allocation between somatic and reproductive growth. Both females and males showed similar rates of increase for eviscerated body mass and digestive gland mass relative to mantle length, but the rate of increase for total reproductive organ weight relative to mantle length in females was three times that of males. For females, condition of somatic tissues deteriorated until the mature stage, but somatic condition improved after the onset of maturity. In males, there was no correlation between somatic condition and phases of reproductive maturity. Reproductive investment decreased as sexual maturation progressed for both females and males, with the lowest investment occurring at the functionally mature stage. Residual analysis indicated that female reproductive development was at the expense of body muscle growth during the immature and maturing stages, but energy invested in reproduction after onset of maturity was probably met by food intake. However, in males both reproductive maturation and somatic growth proceeded concurrently so that energy allocated to reproduction was related to food intake throughout the process of maturation. For both males and females, there was little evidence of trade‐offs between the digestive gland and reproductive growth, as no significant correlation was found between dorsal mantle length‐digestive gland weight residuals. The role of the digestive gland as an energy reserve for gonadal growth should be reconsidered. Additionally, feeding intensity by both males and females decreased after the onset of sexual maturity, but feeding never stopped completely, even during spawning.     

Fitness of alternative modes of reproduction: developmental constraints and the evolutionary maintenance of sex

Parthenogenesis, including facultative parthenogenesis, is common among orthopteroid insects. We investigated the fitness associated with sexual and asexual reproduction within a population of the facultatively parthenogenetic cockroach Nauphoeta cinerea. There is significantly reduced fitness for females reproducing parthenogenetically compared to sexually. Fewer than half of all females can reproduce parthenogenetically. In addition, tenfold fewer offspring are produced by parthenogenesis due to reductions in both the number of offspring produced per clutch and the number of clutches produced. Development and brooding of sexually or parthenogenetically produced first instar nymphs does not differ, although the production of the first parthenogenetic clutch is delayed relative to the first sexually produced clutch. The fitness of parthenogens is also lower than the fitness of sexually produced offspring. Parthenogens are less viable than sexually produced offspring even in the benign conditions of the laboratory. Development to adulthood of parthenogens is slower. Fewer parthenogens survive to adulthood and the adult life span of parthenogens is reduced. Individuals produced by parthenogenetic reproduction are unlikely to reproduce parthenogenetically themselves. Finally, parthenogenetically produced females produce fewer offspring by sexual reproduction than do sexually produced females. Since parthenogenetic reproduction is apomictic in N. cinerea and parthenogens are diploid, we suggest that asexual reproduction is developmentally constrained. Once meiosis has evolved, returning to a mitotic mode of reproduction may be difficult. Nauphoeta cinerea offers a system for testing how asexuality is constrained as modes of reproduction can be compared within a facultative parthenogen.     

Differential susceptibility to food stress in neonates of sexual and asexual mollies (<Emphasis Type="Italic">Poecilia</Emphasis>, Poeciliidae)

The maintenance of sex is still an evolutionary puzzle given its immediate costs. Stably coexisting complexes of asexually and sexually reproducing forms allow to study mechanisms that balance the costs and benefits of both asexual and sexual reproduction. Here, we tested whether coexisting asexual and sexual fish of the genus Poecilia differed in neonate mortality when exposed to environmental stress in the form of fluctuating temperatures and food deprivation. We find that asexual Amazon mollies, Poecilia formosa, are significantly more sensitive to food stress than their sexual relative Poecilia latipinna, but both are equally unaffected by variable temperatures. Differences in the susceptibility to environmental stress may contribute to diminishing the asexuals’ benefits of a higher intrinsic population growth rate and thus mediate stable coexistence of the two reproductive forms.     

Sex-specific life history patterns in bluegill (Lepomis macrochirus): interacting mechanisms influence individual body size

The ultimate body size that an individual fish achieves can be a function both of direct effects of growth or indirect effects associated with the timing of sexual maturation (and associated energetic tradeoffs). These alternatives are often invoked to explain variation in body size within and among fish populations, but have rarely been considered simultaneously. We assessed how resource availability and timing of maturation interact to influence individual body size of bluegill (Lepomis macrochirus). Resource availability (high and low food) and the social structure of the population (presence or absence of large, mature males) were varied in experimental ponds. Food ration affected growth (larger fish in the high food treatments) and the social structure of the population affected timing of maturation (early maturation of males in the absence of large males). Treatment effects, however, were sex-specific; males responded to the social structure of the population and females were more responsive to resource availability. We also found individuals that became sexually mature were smaller than those that remained immature, although results were sex-specific and resource dependent. For males, individuals that matured were smaller when resources were limited; mature and immature females showed no difference in body size regardless of food ration. We show that both resource availability and the processes that control timing of maturation interact in sex-specific ways to influence body size of bluegill. These results suggest that a more robust explanation for variable body size requires consideration of sex-specific interactions between ecological (food and growth) and evolutionary (timing of maturation) mechanisms.     

Population genetics of the fissiparous holothurian Stichopus chloronotus (Aspidochirotida) on the Great Barrier Reef, Australia

 Population genetic structure was studied in one nearshore and two offshore populations of Stichopus chloronotus, a common holothurian species on Indo-Pacific coral reefs. Genetic variation at five polymorphic loci was examined using allozyme electrophoresis. The nearshore population consisted almost exclusively of male individuals, and more males than females were found in all populations studied. Deviations of heterozygosity from that predicted under Hardy-Weinberg equilibrium indicated that asexual reproduction occurred in all populations. Estimates of the level of asexual reproduction using the ratios of the number of sexually produced individuals to sample size, observed genotypic diversity to expected genotypic diversity, and number of genotypes to sample size confirmed that this reproductive mode was more important at the nearshore reef compared to the two offshore reefs. There were large differences in genotypic frequencies between males and females. F-statistics on clonal genotypic frequencies were not statistically significant between populations for neither females or males, suggesting high dispersal of larvae between reefs. A higher mortality of females during larval or early post-settlement stages, or reduced dispersal capability of female larvae are the most likely reasons for biased sex ratios. Accepted: 23 November 1998     

Platelet serotonin levels and gonadal hormones in rats

The role of gonadal hormones in the control of platelet serotonin levels was studied by evaluating the effect of sexual maturation in rats of both sexes and the time-course of changes following gonadectomy performed either prepubertally or on sexually mature animals. In males, platelet serotonin levels remained fairly stable during sexual maturation as well as during the whole postgonadectomy period monitored (four months). In females, somewhat higher values of platelet serotonin levels in adult than in sexually immature animals were found (9%, p less than 0.001, N = 34). A slight decrease of platelet serotonin (10-18%, p less than 0.05) was observed following ovariectomy of sexually mature females, but it was of transient nature. When females were ovariectomized prepubertally a tendency towards permanently lower platelet serotonin levels was noticed. These results suggest that gonadal hormones have no major role in the control of platelet serotonin levels in rats, although a subtle hormonal modulation of this platelet variable in females may exist.     

Demographic Strategy in a Brown Trout Population in Western Norway1

(1) The individuals in the densely populated stock of brown trout, Salmo trutta L., living in Lake Lane and Stranda River, are small, mature at an early age, and have a relatively short life span. Such a demographic strategy appears favourable for populations having a large, but physically unstable spawning and nursery area, intense competition for food, and little vertebrate predation except for human exploitation. (2) For females, the age of sexual maturation appears largely dependent on growth-rate, size, and survival-rate of the fish. Males do not appear to have the advantage of a corresponding large body size. They mature when smaller and at more varying ages than the females, although the two sexes have almost identical growth-rates. (3) Sexually mature fish have higher condition coefficients than immature ones, which is probably an adaptation to ensure gonadal development and enhance the prospect of reproductive success. (4) Mortality appears to be largely dependent on the age when the fish achieve sexual maturation. Most individuals seem to die after their first spawning season. Females show a delay of one year in their sexual maturation relative to males. This adaptation gives protection to the females.     

Male mate choice based on ontogenetic colour changes of females in the damselfly <Emphasis Type="Italic">Ischnura senegalensis</Emphasis>

While male mate choice behaviour has been reported in many taxa, little is known about its plasticity and evolutionary consequences. In the damselfly Ischnura senegalensis, females exhibit colour dimorphism (gynomorph and andromorph). The body colour of gynomorphs changed ontogenetically in accordance with sexual maturation, while little change occurred in andromorphs. To test the male mate choice between sexually immature and mature females of both morphs, binary choice experiments were conducted. Virgin males that were reared separately from females after emergence did not show significant preference between sexually immature and mature females for both morphs, indicating that virgin males were unable to discriminate female reproductive status. On the other hand, males that had experienced copulation with gynomorphs preferred sexually mature gynomorphs to sexually immature ones. However, males that had experienced copulation with andromorphs could not discriminate between sexually immature and mature andromorphs, probably due to the absence of significant ontogenetic change in their thoracic colour. Therefore, female body colour is an important cue for males in discriminating between sexual maturation stages. Learned mate discrimination depending on copulation experience might help males to detect potential mates effectively and avoid sexually unreceptive immature female. We finally discuss the adaptive significance of the ontogenetic colour change in females.     

BODY SIZE AND HAREM SIZE IN MALE RED-WINGED BLACKBIRDS: MANIPULATING SELECTION WITH SEX-SPECIFIC FEEDERS

We experimentally manipulated the strength of selection in the field on red-winged blackbirds (Agelaius phoeniceus) to test hypotheses about contrasting selective forces that favor either large or small males in sexually size dimorphic birds. Selander (1972) argued that sexual selection favors larger males, while survival selection eventually stabilizes male size because larger males do not survive as well as smaller males during harsh winters. Searcy (1979a) proposed instead that sexual selection may be self limiting: male size might be stabilized not by overwinter mortality, but by breeding-season sexual selection that favors smaller males. Under conditions of energetic stress, smaller males should be able to display more and thus achieve higher reproductive success. Using feeders that provisioned males or females but not both, we produced conditions that mimicked the extremes of natural conditions. We found experimental support for the hypothesis that when food is abundant, sexual selection favors larger males. But even under conditions of severe energetic stress, smaller males did not gain larger harems, as the self-limiting hypothesis predicted. Larger males were more energetically stressed than smaller males, but in ways that affected their future reproductive output rather than their current reproductive performance. Stressed males that returned had smaller wings and tails than those that did not return; among returning stressed males, relative harem sizes were inversely related to wing and tail length. Thus, male body size may be stabilized not by survival costs during the non-breeding season, nor by energetic costs during the breeding season, but by costs of future reproduction that larger males pay for their increased breeding-season effort.     

On the fate of sexual traits under asexuality

Environmental shifts and life‐history changes may result in formerly adaptive traits becoming non‐functional or maladaptive. In the absence of pleiotropy and other constraints, such traits may decay as a consequence of neutral mutation accumulation or selective processes, highlighting the importance of natural selection for adaptations. A suite of traits are expected to lose their adaptive function in asexual organisms derived from sexual ancestors, and the many independent transitions to asexuality allow for comparative studies of parallel trait maintenance versus decay. In addition, because certain traits, notably male‐specific traits, are usually not exposed to selection under asexuality, their decay would have to occur as a consequence of drift. Selective processes could drive the decay of traits associated with costs, which may be the case for the majority of sexual traits expressed in females. We review the fate of male and female sexual traits in 93 animal lineages characterized by asexual reproduction, covering a broad taxon range including molluscs, arachnids, diplopods, crustaceans and eleven different hexapod orders. Many asexual lineages are still able occasionally to produce males. These asexually produced males are often largely or even fully functional, revealing that major developmental pathways can remain quiescent and functional over extended time periods. By contrast, for asexual females, there is a parallel and rapid decay of sexual traits, especially of traits related to mate attraction and location, as expected given the considerable costs often associated with the expression of these traits. The level of decay of female sexual traits, in addition to asexual females being unable to fertilize their eggs, would severely impede reversals to sexual reproduction, even in recently derived asexual lineages. More generally, the parallel maintenance versus decay of different trait types across diverse asexual lineages suggests that neutral traits display little or no decay even after extended periods under relaxed selection, while extensive decay for selected traits occurs extremely quickly. These patterns also highlight that adaptations can fix rapidly in natural populations of asexual organisms, in spite of their mode of reproduction.     

Evolution as a critical component of plankton dynamics

Microevolution is typically ignored as a factor directly affecting ongoing population dynamics. We show here that density-dependent natural selection has a direct and measurable effect on a planktonic predator-prey interaction. We kept populations of Brachionus calyciflorus, a monogonont rotifer that exhibits cyclical parthenogenesis, in continuous flow-through cultures (chemostats) for more than 900 days. Initially, females frequently produced male offspring, especially at high population densities. We observed rapid evolution, however, towards low propensity to reproduce sexually, and by 750 days, reproduction had become entirely asexual. There was strong selection favouring asexual reproduction because, under the turbulent chemostat regime, males were unable to mate with females, produced no offspring, and so had zero fitness. In replicated chemostat experiments we found that this evolutionary process directly influenced the population dynamics. We observed very specific but reproducible plankton dynamics which are explained well by a mathematical model that explicitly includes evolution. This model accounts for both asexual and sexual reproduction and treats the propensity to reproduce sexually as a quantitative trait under selection. We suggest that a similar amalgam of ecological and evolutionary mechanisms may drive the dynamics of rapidly reproducing organisms in the wild.     

Masculinization of the X Chromosome in the Pea Aphid

Evolutionary theory predicts that sexually antagonistic mutations accumulate differentially on the X chromosome and autosomes in species with an XY sex-determination system, with effects (masculinization or feminization of the X) depending on the dominance of mutations. Organisms with alternative modes of inheritance of sex chromosomes offer interesting opportunities for studying sexual conflicts and their resolution, because expectations for the preferred genomic location of sexually antagonistic alleles may differ from standard systems. Aphids display an XX/X0 system and combine an unusual inheritance of the X chromosome with the alternation of sexual and asexual reproduction. In this study, we first investigated theoretically the accumulation of sexually antagonistic mutations on the aphid X chromosome. Our results show that i) the X is always more favourable to the spread of male-beneficial alleles than autosomes, and should thus be enriched in sexually antagonistic alleles beneficial for males, ii) sexually antagonistic mutations beneficial for asexual females accumulate preferentially on autosomes, iii) in contrast to predictions for standard systems, these qualitative results are not affected by the dominance of mutations. Under the assumption that sex-biased gene expression evolves to solve conflicts raised by the spread of sexually antagonistic alleles, one expects that male-biased genes should be enriched on the X while asexual female-biased genes should be enriched on autosomes. Using gene expression data (RNA-Seq) in males, sexual females and asexual females of the pea aphid, we confirm these theoretical predictions. Although other mechanisms than the resolution of sexual antagonism may lead to sex-biased gene expression, we argue that they could hardly explain the observed difference between X and autosomes. On top of reporting a strong masculinization of the aphid X chromosome, our study highlights the relevance of organisms displaying an alternative mode of sex chromosome inheritance to understanding the forces shaping chromosome evolution.     

How to go extinct by mating too much: population consequences of male mate choice and efficiency in a sexual–asexual species complex

Selection acting on individuals is not predicted to maximize population persistence, yet examples that explicitly quantify conflicts between individual and population level benefits are scarce. One such conflict occurs over sexual reproduction because of the cost of sex: sexual populations that suffer the cost of producing males have only half the growth rate compared to asexuals. Male behaviour can additionally impact population dynamics in a variety of ways, and here we study an example where the impact is unusually clear: the riddle of persistence of sperm‐dependent sexual–asexual species complexes. Here, a sexually reproducing host species coexists with an ameiotically reproducing all‐female sperm parasite. Sexual–asexual coexistence should not be stable because the proportion of asexually reproducing females will rapidly increase and the relative abundance of the sexually reproducing host species will decline. A severe shortage of males will lead to sperm limitation for sexual and asexual females and the system collapses. Male mate choice could reduce the reproductive potential of the asexual species and thus potentially prevent the collapse. In the gynogenetic (sperm‐dependent parthenogenetic) Amazon molly Poecilia formosa and its host (P. latipinna or P. mexicana), males discriminate against asexual females to some extent. Using a population‐dynamical model, we examine the population dynamics of this species complex with varying strengths of male discrimination ability and efficiency with which they locate females and produce sperm. The sexual species would benefit from stronger discrimination, thus preventing being displaced by the asexual females. However, males would be required to evolve preferences that are probably too strong to be purely based upon selection acting on individuals. We conclude that male behaviour does not fully prevent but delays extinction, yet this is highly relevant because low local extinction rates strongly promote coexistence as a metapopulation.     

Genetic variation and asexual reproduction in the facultatively parthenogenetic cockroach Nauphoeta cinerea: implications for the evolution of sex

Asexual reproduction could offer up to a two‐fold fitness advantage over sexual reproduction, yet higher organisms usually reproduce sexually. Even in facultatively parthenogenetic species, where both sexual and asexual reproduction is sometimes possible, asexual reproduction is rare. Thus, the debate over the evolution of sex has focused on ecological and mutation‐elimination advantages of sex. An alternative explanation for the predominance of sex is that it is difficult for an organism to accomplish asexual reproduction once sexual reproduction has evolved. Difficulty in returning to asexuality could reflect developmental or genetic constraints. Here, we investigate the role of genetic factors in limiting asexual reproduction in Nauphoeta cinerea, an African cockroach with facultative parthenogenesis that nearly always reproduces sexually. We show that when N. cinerea females do reproduce asexually, offspring are genetically identical to their mothers. However, asexual reproduction is limited to a nonrandom subset of the genotypes in the population. Only females that have a high level of heterozygosity are capable of parthenogenetic reproduction and there is a strong familial influence on the ability to reproduce parthenogenetically. Although the mechanism by which genetic variation facilitates asexual reproduction is unknown, we suggest that heterosis may facilitate the switch from producing haploid meiotic eggs to diploid, essentially mitotic, eggs.     

Within‐population covariation between sexual reproduction and susceptibility to local parasites

Evolutionary biology has yet to reconcile the ubiquity of sex with its costs relative to asexual reproduction. Here, we test the hypothesis that coevolving parasites maintain sex in their hosts. Specifically, we examined the distributions of sexual reproduction and susceptibility to local parasites within a single population of freshwater snails (Potamopyrgus antipodarum). Susceptibility to local trematode parasites (Microphallus sp.) is a relative measure of the strength of coevolutionary selection in this system. Thus, if coevolving parasites maintain sex, sexual snails should be common where susceptibility is high. We tested this prediction in a mixed population of sexual and asexual snails by measuring the susceptibility of snails from multiple sites in a lake. Consistent with the prediction, the frequency of sexual snails was tightly and positively correlated with susceptibility to local parasites. Strikingly, in just two years, asexual females increased in frequency at sites where susceptibility declined. We also found that the frequency of sexual females covaries more strongly with susceptibility than with the prevalence of Microphallus infection in the field. In linking susceptibility to the frequency of sexual hosts, our results directly implicate spatial variation in coevolutionary selection in driving the geographic mosaic of sex.     

Effects of food restriction and social cues on sexual maturation and growth in male musk shrews (Suncus murinus)

In the first experiment, the effects of food availability on growth and reproduction were assessed by placing male musk shrews on 3 different feeding schedules, i.e. fed ad libitum, fed 50% of the amount eaten by the ad-libitum controls or fed 25% of the amount eaten by the ad-libitum controls. After 25 days, growth and sexual maturation were significantly retarded in both of the food-restricted groups compared to the control group. In the second experiment, the combined effects of food availability and social cues were investigated in juvenile males that were either fed ad libitum (during the day) or placed on a restricted diet known to inhibit growth and reproduction. Half of these males were either housed alone or with an adult female (separated from female during the day by a wire barrier, but in full contact at night). The ad-libitum-fed males living with a female ate more food, gained more body weight and were more sexually mature than ad-libitum-fed animals living alone. Furthermore, males in the two food-restricted groups had lower body weights and were sexually immature compared with males in both of the ad-libitum-fed groups. Finally, there were little or no differences in growth and reproduction between animals in the two food-restricted groups. These results suggest that food availability is an important environmental regulator of the timing of sexual maturation, and that stimulatory social cues cannot override the inhibitory effects of an inadequate diet.     

Reproductive modes in the epixylic hepatic Anastrophyllum hellerianum

The frequency and dynamics of sexual and asexual reproduction were investigated in a dioecious epixylic hepatic, Anastrophyllum hellerianum, which has declined in recent decades in Finland as a consequence of forestry practices. In our investigation asexual reproduction by gemmae was the dominant mode of reproduction and specialised gemmiparous shoots were present in all colonies studied. The proportions of dead shoots were considerably higher among sex-expressing than among non-sexexpressing shoots. Our results suggest that lower reproductive investment is required for asexual than for sexual reproduction. For instance, no trade-off is detected between asexual reproduction and survival of the gemmiparous shoots in A. hellerianum. Sexual reproduction occurred only in 12% of the colonies and it was promoted by the following factors: medium shoot density, high proportion of sex-expressing shoots, an even sex ratio and very short distances between individuals representing opposite sexes. The ratio of dead males to dead females was significantly female-biased, which suggests higher mortality among female shoots. At the level of individual shoots, more spores than gemmae were produced. However, as a consequence of the low frequency of sporophyte-bearing shoots, gemma production highly exceeded spore production at the colony level. Furthermore, cultivation tests of the propagules showed that gemmae germinate faster than spores.     

Growth, maturation and reproductive investment in Arctic charr

Size and rates of growth in a cohort of 1 + Arctic charr housed in standard conditions were tracked over 12 months (December to December) and mature and immature males and females compared retrospectively. In both sexes, maturing fish were larger than non-maturing ones. In males, this size differential was the result of differences in growth in winter and early spring, but not in the remainder of the study period. In females, size differentials resulted mainly from growth rate differences immediately prior to breeding. In females but not in males, gonadosomatic index was predicted by growth rates in the months leading up to maturation, and among the females that matured, faster growing fish produced more eggs. Lipid reserves in July were correlated negatively with growth during the previous 7 months and, in females only, lipid reserves were significantly lower in maturing fish than in non-maturing fish, indicating that mobilization of lipid energy reserves in maturing fish had commenced by this time. Variation in investment in gonadal tissue, measured as gonadosomatic index, was not explained by variation in July lipid reserves for either males or females. However, July lipid reserves were negatively correlated with egg number, so females investing more in ova exhibited greater depletion of lipid reserves. These results are discussed in the context of the relationship between body condition and the onset of maturation in salmonids, relative investment in reproduction and sexual differences in the cost of reproduction.     

Biochemical composition of the Atlantic bonito Sarda sarda from the Aegean Sea (eastern Mediterranean Sea) in different stages of sexual maturity

The content (% wet mass) in water, ash, lipid, crude protein, DNA and RNA of different tissues was determined during sexual maturation of bonitos Sarda sarda from the Aegean Sea. A total of 220 specimens were collected in the following stages of sexual maturity: immature, resting, developing, mature, spawning and spent. Highest lipid levels in the white muscle, red muscle and liver were measured in immature specimens, while lowest levels were found in spawning bonitos. The gradual percentage of lipid reduction from immature to spawning bonitos was relatively higher in the liver (females 71·2% and males 64·4%) than in the white (females 59·2% and males 53·5%) and red (females 62·1% and males 51·7%) muscle. Lipid levels in the gonads increased gradually from the immature to spawning stage. The decrease of lipid in the somatic tissues was more intense in females than in males, and gonadal lipid content was higher in females than in males. There was a strong reverse correlation between water and lipid percentage in all tissues. Protein content decreased significantly only in spawning bonitos. The percentage of protein reduction from immature to spawning stage was relatively higher in males than in females in both white (females 3·4% and males 4·6%) and red (females 4·6% and males 5·1%) muscles. Protein content in the liver was significantly lower than in the other tissues, being highest in mature females. Gonadal protein content in females increased with maturation and decreased after spawning. The content in ash exhibited considerable stability. The RNA:DNA ratio exhibited a similar pattern of variation in both muscles. The RNA:DNA ratio increased during gonadal development gradually from the developing to spent stage. It was concluded that in S. sarda during gonadal development, there was an increase in gonadal lipid accompanied by a decrease in somatic tissue lipid reserves. Thus, reproductive inactive bonitos have more lipid in their edible part and a higher nutritional value than active ones.