229
A. Ostrovsky, Evolution of Sexual Reproduction in Marine Invertebrates: Example of gymnolaemate bryozoans,
DOI 10.1007/978-94-007-7146-8_3, © Springer Science+Business Media Dordrecht 2013
Abstract
This chapter contains an analysis of the main directions in the evolution of sexual reproduction
in bryozoans – changes in modes of oogenesis and fertilization, the transition from planktotrophy to a non-feeding larva and its consequences, the origin of embryo incubation, and
the repeated evolution of matrotrophy and placental analogues. The trends that emerge from
this analysis are compared with reproductive analogues in the evolution of the bryozoan
order Ctenostomata as well as other marine invertebrate groups (predominantly echinoderms, molluscs and polychaetes). The conditions under which the cheilostomes radiated in
the Late Cretaceous are considered in detail, and the consequences of the transitions to new
reproductive patterns are analyzed. It is suggested that a shift in oogenesis (reduction in egg
number and increase in their size) and parental care can apparently evolve in Cheilostomata
sequentially, with a short time lag: oogenesis becomes modifi ed fi rst, with the decrease in
the number of offspring compensated soon after by the origin of brooding. Finally, the
stages in the evolution of sexual reproduction in other bryozoan groups (classes
Phylactolaemata and Stenolaemata) are reconstructed.
Keywords
Brooding • Evolution • Fertilization • Lecithotrophy • Oogenesis • Placentotrophy
• Planktotrophy
Evolution of Reproductive Patterns
in Cheilostomata
3
The evolution of skeletal brood chambers (ovicells) in
cheilostome bryozoans coincided with the onset of their
radiation in the Late Cretaceous, which has been attributed to
the origin of the new larval type (Taylor 1988a ). The transition from a planktotrophic to a lecithotrophic larva was
apparently due to a shift in oogenesis. In extant species of the
most ancient cheilostome families (suborder Malacostegina)
there is no brooding and numerous small oligolecithal eggs
after internal fertilization are spawned to the water column
where they develop into feeding cyphonautes larvae (sexual
reproduction pattern I). In contrast, living cheilostome
brooders produce relatively few macrolecithal oocytes that
are generally larger than the oocytes of broadcasting cheilostomes and accumulate a suffi cient amount of nutrient
reserves to complete larval development without feeding
(pattern II). Thus, a shift from oligo- to macrolecithal
oogenesis should be a necessary precondition for the evolution of a non-feeding larva. Further changes led to the origin
of extraembryonic nutrition and placental analogues in some
cheilostome lineages. Finally, most resources were allocated
to the larva at the stage of embryogenesis, and oogenesis
changed back again from macro- to oligolecithal (as demonstrated in species with patterns IV and III).
The discussion that follows deals with the evolution
of advanced patterns of sexual reproduction, accompanied by shifts in oogenesis, multiple origins of embryonic incubation and the evolution of the endotrophic
larva in cheilostome bryozoans. Questions of particular
interest in this respect are the prerequisites and the early
stages of the transition to endotrophy, a connection, if
any, between the change in oogenesis mode and the origin of
brooding, and the role of fertilization and extraembryonic nutrition in further transformations of the reproductive patterns.
A. Ostrovsky, Evolution of Sexual Reproduction in Marine Invertebrates: Example of gymnolaemate bryozoans,
DOI 10.1007/978-94-007-7146-8_3, © Springer Science+Business Media Dordrecht 2013
Abstract
This chapter contains an analysis of the main directions in the evolution of sexual reproduction
in bryozoans – changes in modes of oogenesis and fertilization, the transition from planktotrophy to a non-feeding larva and its consequences, the origin of embryo incubation, and
the repeated evolution of matrotrophy and placental analogues. The trends that emerge from
this analysis are compared with reproductive analogues in the evolution of the bryozoan
order Ctenostomata as well as other marine invertebrate groups (predominantly echinoderms, molluscs and polychaetes). The conditions under which the cheilostomes radiated in
the Late Cretaceous are considered in detail, and the consequences of the transitions to new
reproductive patterns are analyzed. It is suggested that a shift in oogenesis (reduction in egg
number and increase in their size) and parental care can apparently evolve in Cheilostomata
sequentially, with a short time lag: oogenesis becomes modifi ed fi rst, with the decrease in
the number of offspring compensated soon after by the origin of brooding. Finally, the
stages in the evolution of sexual reproduction in other bryozoan groups (classes
Phylactolaemata and Stenolaemata) are reconstructed.
Keywords
Brooding • Evolution • Fertilization • Lecithotrophy • Oogenesis • Placentotrophy
• Planktotrophy
Evolution of Reproductive Patterns
in Cheilostomata
3
The evolution of skeletal brood chambers (ovicells) in
cheilostome bryozoans coincided with the onset of their
radiation in the Late Cretaceous, which has been attributed to
the origin of the new larval type (Taylor 1988a ). The transition from a planktotrophic to a lecithotrophic larva was
apparently due to a shift in oogenesis. In extant species of the
most ancient cheilostome families (suborder Malacostegina)
there is no brooding and numerous small oligolecithal eggs
after internal fertilization are spawned to the water column
where they develop into feeding cyphonautes larvae (sexual
reproduction pattern I). In contrast, living cheilostome
brooders produce relatively few macrolecithal oocytes that
are generally larger than the oocytes of broadcasting cheilostomes and accumulate a suffi cient amount of nutrient
reserves to complete larval development without feeding
(pattern II). Thus, a shift from oligo- to macrolecithal
oogenesis should be a necessary precondition for the evolution of a non-feeding larva. Further changes led to the origin
of extraembryonic nutrition and placental analogues in some
cheilostome lineages. Finally, most resources were allocated
to the larva at the stage of embryogenesis, and oogenesis
changed back again from macro- to oligolecithal (as demonstrated in species with patterns IV and III).
The discussion that follows deals with the evolution
of advanced patterns of sexual reproduction, accompanied by shifts in oogenesis, multiple origins of embryonic incubation and the evolution of the endotrophic
larva in cheilostome bryozoans. Questions of particular
interest in this respect are the prerequisites and the early
stages of the transition to endotrophy, a connection, if
any, between the change in oogenesis mode and the origin of
brooding, and the role of fertilization and extraembryonic nutrition in further transformations of the reproductive patterns.