Gender separation increases somatic growth in females but does not affect lifespan in Nothobranchius furzeri

Abstract

According to life history theory, physiological and ecological traits and parameters influence an individual's life history and thus, ultimately, its lifespan. Mating and reproduction are costly activities, and in a variety of model organisms, a negative correlation of longevity and reproductive effort has been demonstrated. We are employing the annual killifish Nothobranchius furzeri as a vertebrate model for ageing. N. furzeri is the vertebrate displaying the shortest known lifespan in captivity with particular strains living only three to four months under optimal laboratory conditions. The animals show explosive growth, early sexual maturation and age-dependent physiological and behavioural decline. Here, we have used N. furzeri to investigate a potential reproduction-longevity trade-off in both sexes by means of gender separation. Though female reproductive effort and offspring investment were significantly reduced after separation, as investigated by analysis of clutch size, eggs in the ovaries and ovary mass, the energetic surplus was not reallocated towards somatic maintenance. In fact, a significant extension of lifespan could not be observed in either sex. This is despite the fact that separated females, but not males, grew significantly larger and heavier than the respective controls. Therefore, it remains elusive whether lifespan of an annual species evolved in periodically vanishing habitats can be prolonged on the cost of reproduction at all.

Figure 1. Survival rates for males and females of the Nothobranchius furzeri strain MZM-04/10.

Animals were maintained in 40 l tanks as males-only (A) or females-only groups (B) and in mixed-sex control groups. The graphs represent the lifespan of control males (n = 22), males-only (n = 28), control females (n = 24) and females-only (n = 30) recorded during 2 independent trials. Median (50% survival) and maximum lifespan (10% survival) is indicated.

Figure 2. Age dependent changes in mean body weight and size in gender separated and control animals.

Determination of length (A) and body weight (B) of males-only and control males (left) and females-only and control females (right). Graphs show mean ± sem. Significance for given data points were calculated by Bonferroni post hoc test.

Figure 3. Analysis of reproductive effort.

(A) Eggs laid into spawning boxes per female per week were recorded in a female-only (n = 12) and a control female cohort (n = 6) over 5 weeks beginning at the age of 5 weeks. Presented as inset are eggs laid per female per week when the tank bottom was 100% covered with breeding substrate. (B) At the age of 14 weeks ovaries of 4 females from each group were dissected and weighed. In both groups animals were observed with one ovary of smaller size (controls: 1, females-only: 2). These were included in the analysis. (C) Number of eggs in the ovary was counted in both groups (number of ovaries = 4). For this analysis only normal sized ovaries were used. Box plots show median, interquartile range and total range by lines, boxes and whiskers, respectively.