A coordinated sequence of distinct flagellar waveforms enables a sharp flagellar turn mediated by squid sperm pH-taxis

Swimming ability and flagellar motility of sperm packets of the volvocine green alga Pleodorina starrii

Eukaryotic flagella collectively form metachronal waves that facilitate the ability to cause flow or swim. Among such flagellated and planktonic swimmers, large volvocine genera such as Eudorina, Pleodorina and Volvox form bundles of small male gametes (sperm) called "sperm packets" for sexual reproduction. Although these sperm packets reportedly have flagella and the ability to swim, previous studies on volvocine motility have focused on asexual forms and the swimming characteristics of sperm packets remain unknown. However, it is important to quantify the motility of sperm packets and sperm in order to gain insights into the significance of motility in the sexual reproduction of planktonic algae. In this study, we quantitatively described the behavior of three flagellated forms of a male strain of Pleodorina starrii-asexual colonies, sperm packets, and single dissociated sperm-with emphasis on comparison of the two multicellular forms. Despite being smaller, sperm packets swam approximately 1.4 times faster than the asexual colonies of the same male strain. Body length was approximately 0.5 times smaller in the sperm packets than in asexual colonies. The flagella from sperm packets and asexual colonies showed asymmetric waveforms, whereas those from dissociated single sperm showed symmetric waveforms, suggesting the presence of a switching mechanism between sperm packets and dissociated sperm. Flagella from sperm packets were approximately 0.5 times shorter and had a beat period approximately twice as long as those from asexual colonies. The flagella of sperm packets were densely distributed over the anterior part of the body, whereas the flagella of asexual colonies were sparse and evenly distributed. The distribution of flagella, but not the number of flagella, appear to illustrate a significant difference in the speeds of sperm packets and asexual colonies. Our findings reveal novel aspects of the regulation of eukaryotic flagella and shed light on the role of flagellar motility in sexual reproduction of planktonic algae.

Sperm competition risk affects ejaculate strategy in terms of sperm number but not sperm size in squid

In polygamous species, the mode of sperm storage in females influences evolution of sperm quantitative and qualitative traits because it provides the arena for sperm competition, cryptic female choice and fertilization processes. In this study, we compared ejaculate traits of two squid species, Heterololigo bleekeri and Loligo reynaudii. Both species show dimorphic sperm traits associated with alternative reproductive tactics where consort and sneaker males transfer sperm to different storage sites within a female (on the oviduct and near the mouth, respectively). Due to differences in reproductive behaviours and sperm placement, sperm competition risk is expected to be higher in sneakers than in consorts of both species and higher overall in L. reynaudii. Our results demonstrate that the instantaneous number of released sperm is adjusted to the expected sperm competition risk via an elaborate sperm package. Consort sperm are similar in size; however, sneaker sperm have a significantly longer flagellum in H. bleekeri than in L. reynaudii, most likely due to intra-tactic conflicts associated with sperm storage conditions. From consideration of the different mating tactics, we suggest that while levels of sperm competition determine quantitative traits, sperm quality traits are determined more by the mode of sperm storage and fertilization.

Male Alternative Reproductive Tactics and Associated Evolution of Anatomical Characteristics in Loliginid Squid

Loliginid squids provide a unique model system to explore male alternative reproductive tactics (ARTs) and their linkage to size, behavioral decision making, and possibly age. Large individuals fight one another and the winners form temporary consortships with females, while smaller individuals do not engage in male-male agonistic bouts but use various sneaker tactics to obtain matings, each with varying mating and fertilization success. There is substantial behavioral flexibility in most species, as smaller males can facultatively switch to the alternative consort behaviors as the behavioral context changes. These forms of ARTs can involve different: mating posture; site of spermatophore deposition; fertilization success; and sperm traits. Most of the traits of male dimorphism (both anatomical and behavioral) are consistent with traditional sexual selection theory, while others have unique features that may have evolved in response to the fertilization environment faced by each temporary or permanent male morph.

pH sensing by lipids in membranes: The fundamentals of pH-driven migration, polarization and deformations of lipid bilayer assemblies

Most biological molecules contain acido-basic groups that modulate their structure and interactions. A consequence is that pH gradients, local heterogeneities and dynamic variations are used by cells and organisms to drive or regulate specific biological functions including energetic metabolism, vesicular traffic, migration and spatial patterning of tissues in development. While the direct or regulatory role of pH in protein function is well documented, the role of hydrogen and hydroxyl ions in modulating the properties of lipid assemblies such as bilayer membranes is only beginning to be understood. Here, we review approaches using artificial lipid vesicles that have been instrumental in providing an understanding of the influence of pH gradients and local variations on membrane vectorial motional processes: migration, membrane curvature effects promoting global or local deformations, crowding generation by segregative polarization processes. In the case of pH induced local deformations, an extensive theoretical framework is given and an application to a specific biological issue, namely the structure and stability of mitochondrial cristae, is described. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.