3.3 Reproduction
The overwhelming majority of marine ostracods exhibit sexual reproduction,
although some brackish and freshwater species have acquired the ability to reproduce asexually. “(…) sexual propagation may be regarded as a source of individual
variability, furnishing material for the operation of natural selection” (August
Weismann 1887, quoted in Butlin et al. 1998a). Apart from undoubted genetic
benefits of sexual reproduction there are some costs. The main ones are the ‘cost of
males’, the requirement for males in sexual lineages, and the ‘cost of mating’, the
energetic costs of finding partner, courting, copulation, and predation and disease
risk involved in mating (Butlin et al. 1998a).
It is still uncertain why parthenogenesis is so common in non-marine ostracods.
There are a few different forms of ostracod asexuality (Butlin et al. 1998b; Martens
1998). An ancient asexual does not have close sexual relatives, and its populations
consist exclusively of females. Some lineages have geographically restricted sexual
and asexual populations—so-called ‘geographical parthenogenesis’. Finally, there
are populations whose sexual and asexual lineages coexist. The last mode, referred
to as ‘mixed reproduction’, seems to be the most beneficial for new habitat colonisation. Parthenogenesis enables quick and easy dispersion and increase in abundance, because a single egg is sufficient to invade a new water habitat. Alternatively,
in highly changeable brackish and freshwater habitats, the diversity of sexual parents
offspring might have a better chance to adapt and survive. The first ostracods
recorded as freshwater invaders in the Carboniferous probably exhibited mixed
reproduction (Griffiths and Horne 1998; Liebau 2005; Bennett 2008).
Both fossil and modern ostracods can exhibit advanced reproductive strategies
that facilitate survival in new salinity regimes. The production of resting eggs, or
resistance to desiccation or other unfavourable environmental conditions, may also
have been attributes of the first fresh water invaders, for example species of Carbonita from a temporary pond habitat, found in the Montceau Lagerstätte (Vannier
et al. 2003). Brooding may also have allowed the colonisation of more extreme
habitats. In the deposits from Lower Silurian Herefordshire Konservat-Lagerstätte
myodocopan species with eggs and possibly juveniles were preserved, thus providing an unequivocal view of parental brood care as a reproductive strategy which
has lasted within this group from the Silurian to present day (Siveter 2008).
The typical foraminiferal life cycle is characterised by an alteration of asexual
and sexual generations. In the sexual generation the adult gamont produces
gametes, and fertilisation takes place by the fusion of two gametes, usually from
different parents. The zygote may spend a brief phase as a shell-less (naked)
amoeba. In metazoans meiosis typically occurs during gametogenesis, however, in
the asexual generation the foraminiferal agamont produces numerous offspring by
multiple fission, with meiosis as an integral part of this process. Thus, haploid
young individuals typically grow to become adult gamonts, which produce gametes
by mitotic nuclear divisions (Pawlowski 2009).
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A. Iglikowska and J. Pawłowska
The overwhelming majority of marine ostracods exhibit sexual reproduction,
although some brackish and freshwater species have acquired the ability to reproduce asexually. “(…) sexual propagation may be regarded as a source of individual
variability, furnishing material for the operation of natural selection” (August
Weismann 1887, quoted in Butlin et al. 1998a). Apart from undoubted genetic
benefits of sexual reproduction there are some costs. The main ones are the ‘cost of
males’, the requirement for males in sexual lineages, and the ‘cost of mating’, the
energetic costs of finding partner, courting, copulation, and predation and disease
risk involved in mating (Butlin et al. 1998a).
It is still uncertain why parthenogenesis is so common in non-marine ostracods.
There are a few different forms of ostracod asexuality (Butlin et al. 1998b; Martens
1998). An ancient asexual does not have close sexual relatives, and its populations
consist exclusively of females. Some lineages have geographically restricted sexual
and asexual populations—so-called ‘geographical parthenogenesis’. Finally, there
are populations whose sexual and asexual lineages coexist. The last mode, referred
to as ‘mixed reproduction’, seems to be the most beneficial for new habitat colonisation. Parthenogenesis enables quick and easy dispersion and increase in abundance, because a single egg is sufficient to invade a new water habitat. Alternatively,
in highly changeable brackish and freshwater habitats, the diversity of sexual parents
offspring might have a better chance to adapt and survive. The first ostracods
recorded as freshwater invaders in the Carboniferous probably exhibited mixed
reproduction (Griffiths and Horne 1998; Liebau 2005; Bennett 2008).
Both fossil and modern ostracods can exhibit advanced reproductive strategies
that facilitate survival in new salinity regimes. The production of resting eggs, or
resistance to desiccation or other unfavourable environmental conditions, may also
have been attributes of the first fresh water invaders, for example species of Carbonita from a temporary pond habitat, found in the Montceau Lagerstätte (Vannier
et al. 2003). Brooding may also have allowed the colonisation of more extreme
habitats. In the deposits from Lower Silurian Herefordshire Konservat-Lagerstätte
myodocopan species with eggs and possibly juveniles were preserved, thus providing an unequivocal view of parental brood care as a reproductive strategy which
has lasted within this group from the Silurian to present day (Siveter 2008).
The typical foraminiferal life cycle is characterised by an alteration of asexual
and sexual generations. In the sexual generation the adult gamont produces
gametes, and fertilisation takes place by the fusion of two gametes, usually from
different parents. The zygote may spend a brief phase as a shell-less (naked)
amoeba. In metazoans meiosis typically occurs during gametogenesis, however, in
the asexual generation the foraminiferal agamont produces numerous offspring by
multiple fission, with meiosis as an integral part of this process. Thus, haploid
young individuals typically grow to become adult gamonts, which produce gametes
by mitotic nuclear divisions (Pawlowski 2009).
100
A. Iglikowska and J. Pawłowska