Publications

2016
Krauchunas, AR, Marcello MR, Singson "A.  2016.  The molecular complexity of fertilization: Introducing the concept of a fertilization synapse. Molecular Reproduction and Development.
2015
Singaravelu, G, Rahimi S, Krauchunas A, Rizvi A, Dharia S, Shakes D, Smith H, Golden A, Singson A.  2015.  Forward Genetics Identifies a Requirement for the Izumo-like Immunoglobulin Superfamily spe-45 Gene in Caenorhabditis elegans Fertilization.. Current Biology. 25:3220-3224.
2013
Chatterjee, I, Ibanez-Ventoso C, Vijay P, Singaravelu G, Baldi C, Bair J, Ng S, Smolyanskaya A, Driscoll M, Singson A.  2013.  Dramatic fertility decline in aging C. elegans males is associated with mating execution deficits rather than diminished sperm quality. Exp. Gerontol.. 48:1156–1166. Abstract
Although much is known about female reproductive aging, fairly little is known about the causes of male reproductive senescence. We developed a method that facilitates culture maintenance of Caenorhabditis elegans adult males, which enabled us to measure male fertility as populations age, without profound loss of males from the growth plate. We find that the ability of males to sire progeny declines rapidly in the first half of adult lifespan and we examined potential factors that contribute towards reproductive success, including physical vigor, sperm quality, mating apparatus morphology, and mating ability. Of these, we find little evidence of general physical decline in males or changes in sperm number, morphology, or capacity for activation, at time points when reproductive senescence is markedly evident. Rather, it is the loss of efficient mating ability that correlates most strongly with reproductive senescence. Low insulin signaling can extend male ability to sire progeny later in life, although insulin impact on individual facets of mating behavior is complex. Overall, we suggest that combined modest deficits, predominantly affecting the complex mating behavior rather than sperm quality, sum up to block effective C. elegans male reproduction in middle adult life.
Singaravelu, G, Singson A.  2013.  Calcium signaling surrounding fertilization in the nematode Caenorhabditis elegans. Cell Calcium. 53:2-9. Abstract
Calcium plays a prominent role during fertilization in many animals. This review focuses on roles of Ca(2+) during the events around fertilization in the model organism, Caenorhabditis elegans. Specifically, the role of Ca(2+) in sperm, oocytes and the surrounding somatic tissues during fertilization will be discussed, with the focus on sperm activation, meiotic maturation of oocytes, ovulation, sperm-egg interaction and fertilization.
Marcello, MR, Singaravelu G, Singson A.  2013.  Fertilization. Adv. Exp. Med. Biol.. 757:321–350. Abstract
Fertilization-the fusion of gametes to produce a new organism-is the culmination of a multitude of intricately regulated cellular processes. In Caenorhabditis elegans, fertilization is highly efficient. Sperm become fertilization competent after undergoing a maturation process during which they become motile, and the plasma membrane protein composition is reorganized in preparation for interaction with the oocyte. The highly specialized gametes begin their interactions by signaling to one another to ensure that fertilization occurs when they meet. The oocyte releases prostaglandin signals to help guide the sperm to the site of fertilization, and sperm secrete a protein called major sperm protein (MSP) to trigger oocyte maturation and ovulation. Upon meeting one another in the spermatheca, the sperm and oocyte fuse in a specific and tightly regulated process. Recent studies are providing new insights into the molecular basis of this fusion process. After fertilization, the oocyte must quickly transition from the relative quiescence of oogenesis to a phase of rapid development during the cleavage divisions of early embryogenesis. In addition, the fertilized oocyte must prevent other sperm from fusing with it as well as produce an eggshell for protection during external development. This chapter will review the nature and regulation of the various cellular processes of fertilization, including the development of fertilization competence, gamete signaling, sperm-oocyte fusion, the oocyte to embryo transition, and production of an eggshell to protect the developing embryo.
2012
Singaravelu, G, Chatterjee I, Rahimi S, Druzhinina MK, Kang L, Xu XZ, Singson A.  2012.  The sperm surface localization of the TRP-3/SPE-41 Ca2+ -permeable channel depends on SPE-38 function in Caenorhabditis elegans. Dev. Biol.. 365:376–383. Abstract
Despite undergoing normal development and acquiring normal morphology and motility, mutations in spe-38 or trp-3/spe-41 cause identical phenotypes in Caenorhabditis elegans-mutant sperm fail to fertilize oocytes despite direct contact. SPE-38 is a novel, four-pass transmembrane protein and TRP-3/SPE-41 is a Ca(2+)-permeable channel. Localization of both of these proteins is confined to the membranous organelles (MOs) in undifferentiated spermatids. In mature spermatozoa, SPE-38 is localized to the pseudopod and TRP-3/SPE-41 is localized to the whole plasma membrane. Here we show that the dynamic redistribution of TRP-3/SPE-41 from MOs to the plasma membrane is dependent on SPE-38. In spe-38 mutant spermatozoa, TRP-3/SPE-41 is trapped within the MOs and fails to reach the cell surface despite MO fusion with the plasma membrane. Split-ubiquitin yeast-two-hybrid analyses revealed that the cell surface localization of TRP-3/SPE-41 is likely regulated by SPE-38 through a direct protein-protein interaction mechanism. We have identified sequences that influence the physical interaction between SPE-38 and TRP-3/SPE-41, and show that these sequences in SPE-38 are required for fertility in transgenic animals. Despite the mislocalization of TRP-3/SPE-41 in spe-38 mutant spermatozoa, ionomycin or thapsigargin induced influx of Ca(2+) remains unperturbed. This work reveals a new paradigm for the regulated surface localization of a Ca(2+)-permeable channel.
2011
Marcello, MR, Singson A.  2011.  Germline determination: don't mind the P granules.. Curr Biol.. 21(4):R155-7.
Parry, JM, Singson A.  2011.  EGG molecules couple the oocyte-to-embryo transition with cell cycle progression. Results Probl Cell Differ. 53:135–151. Abstract
The oocyte-to-embryo transition is a precisely coordinated process in which an oocyte becomes fertilized and transitions to an embryonic program of events. The molecules involved in this process have not been well studied. Recently, a group of interacting molecules in C. elegans have been described as coordinating the oocyte-to-embryo transition with the advancement of the cell cycle. Genes egg-3, egg-4, and egg-5 represent a small class of regulatory molecules known as protein-tyrosine phosphase-like proteins, which can bind phosphorylated substrates and act as scaffolding molecules or inhibitors. These genes are responsible for coupling the movements and activities of regulatory kinase mbk-2 with advancement of the cell cycle during the oocyte-to-embryo transition.
Geldziler, BD, Marcello MR, Shakes DC, Singson A.  2011.  The genetics and cell biology of fertilization. Methods Cell Biol.. 106:343–375. Abstract
Although the general events surrounding fertilization in many species are well described, the molecular underpinnings of fertilization are still poorly understood. Caenorhabditis elegans has emerged as a powerful model system for addressing the molecular and cell biological mechanism of fertilization. A primary advantage is the ability to isolate and propagate mutants that effect gametes and no other cells. This chapter provides conceptual guidelines for the identification, maintenance, and experimental approaches for the study fertility mutants.
Singaravelu, G, Chatterjee I, Marcello MR, Singson A.  2011.  Isolation and in vitro activation of Caenorhabditis elegans sperm. J Vis Exp. Abstract
Males and hermaphrodites are the two naturally found sexual forms in the nematode C. elegans. The amoeboid sperm are produced by both males and hermaphrodites. In the earlier phase of gametogenesis, the germ cells of hermaphrodites differentiate into limited number of sperm–around 300–and are stored in a small 'bag' called the spermatheca. Later on, hermaphrodites continually produce oocytes. In contrast, males produce exclusively sperm throughout their adulthood. The males produce so much sperm that it accounts for > 50% of the total cells in a typical adult worm. Therefore, isolating sperm from males is easier than from that of hermaphrodites. Only a small proportion of males are naturally generated due to spontaneous non-disjunction of X chromosome. Crossing hermaphrodites with males or more conveniently, the introduction of mutations to give rise to Him (High Incidence of Males) phenotype are some of strategies through which one can enrich the male population. Males can be easily distinguished from hermaphrodites by observing the tail morphology. Hermaphrodite's tail is pointed, whereas male tail is rounded with mating structures. Cutting the tail releases vast number of spermatids stored inside the male reproductive tract. Dissection is performed under a stereo microscope using 27 gauge needles. Since spermatids are not physically connected with any other cells, hydraulic pressure expels internal contents of male body, including spermatids. Males are directly dissected on a small drop of 'Sperm Medium'. Spermatids are sensitive to alteration in the pH. Hence, HEPES, a compound with good buffering capacity is used in sperm media. Glucose and other salts present in sperm media help maintain osmotic pressure to maintain the integrity of sperm. Post-meiotic differentiation of spermatids into spermatozoa is termed spermiogenesis or sperm activation. Shakes, and Nelson previously showed that round spermatids can be induced to differentiate into spermatozoa by adding various activating compounds including Pronase E. Here we demonstrate in vitro spermiogenesis of C. elegans spermatids using Pronase E. Successful spermiogenesis is pre-requisite for fertility and hence the mutants defective in spermiogenesis are sterile. Hitherto several mutants have been shown to be defective specifically in spermiogenesis process. Abnormality found during in vitro activation of novel Spe (Spermatogenesis defective) mutants would help us discover additional players participating in this event.
Singaravelu, G, Singson A.  2011.  New insights into the mechanism of fertilization in nematodes. Int Rev Cell Mol Biol. 289:211–238. Abstract
Fertilization results from the fusion of male and female gametes in all sexually reproducing organisms. Much of nematode fertility work was focused on Caenorhabditis elegans and Ascaris suum. The C. elegans hermaphrodite produces a limited number of sperm initially and then commits to the exclusive production of oocytes. The postmeiotic differentiation called spermiogenesis converts sessile spermatids into motile spermatozoa. The motility of spermatozoa depends on dynamic assembly and disassembly of a major sperm protein-based cytoskeleton uniquely found in nematodes. Both self-derived and male-derived spermatozoa are stored in spermatheca, the site of fertilization in hermaphrodites. The oocyte is arrested in meiotic prophase I until a sperm-derived signal relieves the inhibition allowing the meiotic maturation to occur. Oocyte undergoes meiotic maturation, enters into spermatheca, gets fertilized, completes meiosis, and exits into uterus as a zygote. This review focuses on our current understanding of the events around fertilization in nematodes.