Early embryonic determination of the sexual dimorphism in segment number in geophilomorph centipedes

Carlo Brena,Jack Green,Michael Akam

doi:10.1186/2041-9139-4-22

Abstract

BackgroundMost geophilomorph centipedes show intraspecific variability in the number of leg-bearing segments. This intraspecific variability generally has a component that is related to sex, with females having on average more segments than males. Neither the developmental basis nor the adaptive role of this dimorphism is known.ResultsTo determine when this sexual dimorphism in segment number is established, we have followed the development of Strigamia maritima embryos from the onset of segmentation to the first post-embryonic stage where we could determine the sex morphologically. We find that males and females differ in segment number by Stage 6.1, a point during embryogenesis when segment addition pauses while the embryo undergoes large-scale movements. We have confirmed this pattern by establishing a molecular method to determine the sex of single embryos, utilising duplex PCR amplification for Y chromosomal and autosomal sequences. This confirms that male embryos have a modal number of 43 segments visible at Stage 6, while females have 45. In our Strigamia population, adult males have a modal number of 47 leg-bearing segments, and females have 49. This implies that the sexual dimorphism in segment number is determined before the addition of the last leg-bearing segments and the terminal genital segments.ConclusionsSexual dimorphism in segment number is not associated with terminal segment differentiation, but must instead be related to some earlier process during segment patterning. The dimorphism may be associated with a difference in the rate and/or duration of segment addition during the main phase of rapid segment addition that precedes embryonic Stage 6. This suggests that the adaptive role, if any, of the dimorphism is likely to be related to segment number per se, and not to sexual differentiation of the terminal region.

Highlights

Most geophilomorph centipedes show intraspecific variability in the number of leg-bearing segments
They show extreme variability in adult segment number, with the number of leg-bearing segments ranging from 27 to 191 between species [1]. This number is always odd – a striking example of constraint in natural variation [2]. All of these segments develop during embryogenesis: juveniles hatch with the final adult number of leg-bearing segments already present
To determine whether the variation in segment number at Stage 6.1 is related to the sex of the embryo, a sample of 22 eggs that had been scored for segment number throughout embryogenesis [11] were maintained in culture until the adolescens I stage, the first stage at which it becomes possible to sex individuals with some degree of accuracy by morphological criteria (Figure 1D,E) [5]

Summary

Introduction

Most geophilomorph centipedes show intraspecific variability in the number of leg-bearing segments. Centipedes of the Order Geophilomorpha provide attractive models for studying the specification and evolution of segment number in arthropods They show extreme variability in adult segment number, with the number of leg-bearing segments ranging from 27 to 191 between species [1]. This number is always odd – a striking example of constraint in natural variation [2]. All of these segments develop during embryogenesis: juveniles hatch with the final adult number of leg-bearing segments already present In this respect, geophilomorphs are derived among centipedes, sharing this trait only with their sister group the Scolopendromorpha. Only one basal group, the Mecistocephalidae, do not generally show such a difference between the sexes [8]

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