Germ-cell warfare in ascidians: sperm from one species can interfere with the fertilization of a second species

Non-lethal effects of an invasive species in the marine environment: the importance of early life-history stages

Studies examining the effects of invasive species have focussed traditionally on the direct/lethal effects of the invasive on the native community but there is a growing recognition that invasive species may also have non-lethal effects. In terrestrial systems, non-lethal effects of invasive species can disrupt early life-history phases (such as fertilisation, dispersal and subsequent establishment) of native species, but in the marine environment most studies focus on adult rather than early life-history stages. Here, we examine the potential for an introduced sessile marine invertebrate (Styela plicata) to exert both lethal and non-lethal effects on a native species (Microcosmus squamiger) across multiple early life-history stages. We determined whether sperm from the invasive species interfered with the fertilisation of eggs from the native species and found no effect. However, we did find strong effects of the invasive species on the post-fertilisation performance of the native species. The invasive species inhibited the settlement of native larvae and, in the field, the presence of the invasive species was associated with a ten-fold increase in the post-settlement mortality of the native species, as well as an initial reduction of growth in the native. Our results suggest that larvae of the native species avoid settling near the invasive species due to reduced post-settlement survival in its presence. Overall, we found that invasive species can have complex and pervasive effects (both lethal and non-lethal) across the early life-history stages of the native species, which are likely to result in its displacement and to facilitate further invasion.

Signaling pathways in ascidian oocyte maturation: the role of cyclic AMP and follicle cells in germinal vesicle breakdown

Many ascidian oocytes undergo 'spontaneous' germinal vesicle breakdown (GVBD) when transferred from the ovary to normal pH 8.2 sea water (SW); however, low pH inhibits GVBD, which can then be stimulated while remaining in the low pH SW. Oocytes of Boltenia villosa blocked from GVBD by pH 4 SW undergo GVBD in response to permeant cyclic AMP (8-bromo-cyclic AMP), phosphodiesterase inhibitors (isobutylmethylxanthine and theophylline) or the adenylyl cyclase activator forskolin. This suggests that cAMP increases during GVBD. Removal of the follicle cells or addition of a protease inhibitor inhibits GVBD in response to raised pH but not to forskolin, theophylline or 8 bromo-cAMP. Isolated follicle cells in low pH SW release protease activity in response to an increase in pH. These studies imply that the follicle cells release protease activity, which either itself stimulates an increase in oocyte cAMP level or reacts with other molecules to stimulate this process. Studies with the mitogen-activated protein (MAP) kinase inhibitors U0126 and CI 1040 suggest that MAP kinase is not involved in GVBD. The Cdc25 inhibitor NSC 95397 inhibits GVBD at 200 nm in a reversible manner.

Ecology and natural history of the protochordates

The last comprehensive reviews of ecology and natural history of ascidians were included in the excellent 1971 publication by Millar on the biology of ascidians and the 1991 treatise on New Caledonia ascidians by Monniot, Monniot, and Laboute. Several hundred papers have been published since that time, greatly expanding our knowledge of environmental tolerances and responses to increasing levels of anthropogenically derived toxins in marine waters, energetics and feeding strategies, predator–prey relationships, competition both intra- and inter-specific that include many studies of self–nonself recognition in colonial species, modes and environmental regulation of reproduction and development, symbionts, natural-product chemistry as antifouling and antipredator defenses, and dispersal mechanisms. The relatively new field of molecular genetics is revealing the presence of cryptic species and is helping to determine the origin of anthropogenically transported individuals, an important and growing problem that affects natural ecological relationships in marine communities worldwide. We are learning more about the difficult-to-study abyssal and Antarctic species. There have been great advances in our understanding of the importance in open-ocean food webs of the planktonic Appendicularia and Thaliacea. Also included in this review is a brief discussion of recent work on the Cephalochordata and Hemichordata.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Historical introduction, overview, and reproductive biology of the protochordates

This issue of the Canadian Journal of Zoology exhaustively reviews most major aspects of protochordate biology by specialists in their fields. Protochordates are members of two deuterostome phyla that are exclusively marine. The Hemichordata, with solitary enteropneusts and colonial pterobranchs, share a ciliated larva with echinoderms and appear to be closely related, but they also have many chordate-like features. The invertebrate chordates are composed of the exclusively solitary cephalochordates and the tunicates with both solitary and colonial forms. The cephalochordates are all free-swimming, but the tunicates include both sessile and free-swimming forms. Here I explore the history of research on protochordates, show how views on their relationships have changed with time, and review some of their reproductive and structural traits not included in other contributions to this special issue.

Interspecific competition during transmission of two sympatric malaria parasite species to the mosquito vector

The role of species interactions in structuring parasite communities remains controversial. Here, we show that interspecific competition between two avian malaria parasite species, Plasmodium gallinaceum and P. juxtanucleare, occurs as a result of interference during parasite fertilization within the bloodmeal of the mosquito. The significant reduction in the transmission success of P. gallinaceum to mosquitoes, due to the co-infecting P. juxtanucleare, is predicted to have compromised its colonization of regions occupied by P. juxtanucleare and, thus, may have contributed to the restricted global distribution of P. gallinaceum. Such interspecies interactions may occur between human malaria parasites and, thus, impact upon parasite species epidemiology, especially in regions of seasonal transmission.

Efficiency of gamete usage in nature: sperm storage, fertilization and polyspermy

Gamete production for both males and females can be energetically expensive such that selection should maximize fertilization opportunities while minimizing fertilization costs. In laboratory studies of Drosophila reproduction, however, the failure of eggs to yield adult progeny can be quite high, suggesting that female control over gamete utilization is surprisingly inefficient. We examined gamete utilization in D. pseudoobscura from nature and compared our observations to those for laboratory populations. In natural populations 100% of oviposited eggs effectively produce adult progeny, and fertilization is exclusively monospermic, indicating that in nature, D. pseudoobscura females maintain a very strict control over their reproduction such that gamete usage is extremely efficient. The potential reasons for the inefficient gamete utilization in the laboratory, as well as the potential impact on laboratory studies of sperm competition, sexual conflict, and the evolution of reproductive barriers are discussed. Furthermore, in this sperm-heteromorphic species, our observations show definitively that in nature, as well as in the laboratory, only the long sperm morph participates in fertilization.