Female reproductive patterns in the millipede Polydesmus angustus (Diplopoda: Polydesmidae) and their significance for cohort-splitting

Abstract

Abstract. First-stadium juveniles of Polydesmus angustus born each month from May to September were reared throughout their life cycle under controlled seasonal conditions. At maturity, the reproductive patterns of 62 females were studied individually. It was confirmed that females born from May to August have a 1-year life cycle and those born from late August onwards a 2-year life cycle (cohort-splitting). A third type of life cycle -interseasonal iteroparity -was observed in a few females born late in the season. On average, annual females started to reproduce when 11.4 months old and produced 3.6 broods per female over 1.8 months; the later they were born from May to August, the later they reproduced the following year. Biennial females started to reproduce when 19.9 months old and produced 3.8 broods per female over 2.2 months; all reproduced early in the breeding season. These results indicate that only annual females can produce an appreciable proportion of biennial offspring from late August onwards, which rules out direct genetic determination of life-cycle duration. The reproductive characteristics of P. angustus suggest a non-genetic mechanism that can drive cohort-splitting. Because individual females reproduce for about 2 months on average, this automatically results in cyclic variation in life-cycle duration (annual/biennial/annual) in the long-term progeny of any female.

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

“…The extended life cycle results from a delay in development, due to aestivation in stadium VII, combined with the adult females overwintering in reproductive diapause. In a previous study, it was shown that biennial offspring are produced almost exclusively by annual females, so this pattern cannot be determined genetically (David, 2009). In the present study the hypothesis that the propensity of late-born individuals to aestivate results, at least in part, from a non-genetic maternal effect, was tested.…”

“…This species has a long period of reproduction in spring-summer, during which each female produces several broods at intervals of a few weeks before dying (David, 2009). Eggs are laid in nests made of earthy faecal material, and first stadium juveniles leave the nest soon after hatching.…”

“…In the temperate millipede Polydesmus angustus Latzel, an individual's life cycle depends on its month of birth (David et al, 1999;David, 2009). That of individuals born early in the breeding season (May -August) is annual, and those born later (August -early October) biennial, irrespective of the parental life cycles (David, 2009).…”

“…That of individuals born early in the breeding season (May -August) is annual, and those born later (August -early October) biennial, irrespective of the parental life cycles (David, 2009). Biennial individuals undergo two periods of diapause: (1) they aestivate in the penultimate stadium (stadium VII) during the first breeding season after their birth, and reach maturity in early autumn; (2) adult females that emerge in autumn do not reproduce immediately; they overwinter a second time and reproduce in the second breeding season after birth (David et al, 1999).…”

“…In P. angustus, the occurrence of photoperiodically induced diapause only in late-born offspring suggests that they might be predisposed for longer life cycles. There is no possibility of genetic polymorphism for life-cycle duration in this millipede, because biennial offspring are produced almost exclusively by annual females (David, 2009). However, a non-genetic maternal effect might extend the development of individuals born late in the season.…”