Cohort-splitting in the millipede Polydesmus angustus (Diplopoda: Polydesmidae): No evidence for maternal effects on life-cycle duration

David Jean-François,Jean-Jacques Geoffroy

doi:10.14411/eje.2011.046

Abstract

Under seasonal conditions, Polydesmus angustus individuals born in the first part of the breeding season have a 1-year life cycle and those born later have a 2-year life cycle (cohort-splitting). In this study, 249 juveniles from four early broods (born in mid- July) and four late broods (born in September) were reared under similar laboratory conditions, to test for possible maternal influ- ences on life-cycle duration. Development times of early- and late-born individuals were compared under four combinations of day length and temperature (16 h - 18°C, 16 h - 16°C, 12 h - 18°C and 12 h - 16°C). The results showed that development time varied significantly in response to day length, temperature and sex, but that of individuals in the early and late broods did not differ signifi- cantly (mean development times ± SE: 180 ± 6 and 183 ± 8 days, respectively). There were no significant interactions between birth period and other factors, indicating that the effects of day length, temperature and sex on development time were similar in early- and late-born individuals. This indicates that the extended life cycle of millipedes born late in the season is not maternally deter- mined and that cohort-splitting is controlled entirely by the environmental conditions experienced by the offspring during their development. This conclusion is supported by the absence of significant variation in offspring live weight at birth measured at dif- ferent times in the breeding season. The results are discussed in relation to the bet-hedging theory, which is often put forward to account for cohort-splitting in arthropods. In P. angustus, the results are consistent with either bet-hedging or adaptive plasticity, but further studies are required to decide which interpretation is correct.

Highlights

In many arthropods with relatively long life cycles (t 1 year), the cohort of individuals born in the same breeding season splits into groups that reproduce in different years
Clarifying the mechanism involved is important because cohort-splitting is often interpreted as diversified bet-hedging, i.e. phenotypic variation that spreads the risks of a population crash in unpredictable environments, which is incompatible with genetic polymorphism (Seger & Brockmann, 1987; Stearns, 1992; Hopper, 1999)
Cohort-splitting, i.e. the occurrence of different types of life cycles among individuals born in the same breeding season (Sunderland et al, 1976) is widespread in arthropods

Summary

Introduction

In many arthropods with relatively long life cycles (t 1 year), the cohort of individuals born in the same breeding season splits into groups that reproduce in different years. This phenomenon of cohort-splitting (Sunderland et al, 1976), called parsivoltinism in insects (Torchio & Tepedino, 1982), is reported in woodlice (Sunderland et al, 1976; Zimmer & Kautz, 1997), millipedes (Snider, 1984; David et al, 1993), spiders (Bonte & Maelfait, 2001) and insects, aquatic and terrestrial Bet-hedging is more likely to occur when maternal effects control life-cycle duration a long time in advance, whereas plasticity based on environmental cues during offspring development is more likely to be an adaptation to predictable conditions (Hopper, 1999; Roff, 2002)

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