Germ cell cysts and simultaneous sperm and oocyte production in a hermaphroditic nematode

Abstract

Studies of gamete development in the self-fertile hermaphrodites of Caenorhabditis elegans have significantly contributed to our understanding of fundamental developmental mechanisms. However, evolutionary transitions from outcrossing males and females to self-fertile hermaphrodites have convergently evolved within multiple nematode sub-lineages, and whether the C. elegans pattern of self-fertile hermaphroditism and gamete development is representative remains largely unexplored. Here we describe a pattern of sperm production in the trioecious (male/female/hermaphrodite) nematode Rhabditis sp. SB347 (recently named Auanema rhodensis) that differs from C. elegans in two striking ways. First, while C. elegans hermaphrodites make a one-time switch from sperm to oocyte production, R. sp. SB347 hermaphrodites continuously produce both sperm and oocytes. Secondly, while C. elegans germ cell proliferation is limited to germline stem cells (GSCs), sperm production in R. sp. SB347 includes an additional population of mitotically dividing cells that are a developmental intermediate between GSCs and fully differentiated spermatocytes. These cells are present in males and hermaphrodites but not females, and exhibit key characteristics of spermatogonia - the mitotic progenitors of spermatocytes in flies and vertebrates. Specifically, they exist outside the stem cell niche, increase germ cell numbers by transit-amplifying divisions, and synchronously proliferate within germ cell cysts. We also discovered spermatogonia in other trioecious Rhabditis species, but not in the male/female species Rhabditis axei or the more distant hermaphroditic Oscheius tipulae. The discovery of simultaneous hermaphroditism and spermatogonia in a lab-cultivatable nematode suggests R. sp. SB347 as a richly informative species for comparative studies of gametogenesis.

Sex- and Gamete-Specific Patterns of X Chromosome Segregation in a Trioecious Nematode

“…Previously, we showed that sperm production in A. rhodensis hermaphrodites differs from that in C. elegans , because A. rhodensis hermaphrodites produce sperm from discrete clusters of spermatogonial cells—both simultaneously and continuously along with oocytes [ 15 ]. In addition, A. rhodensis hermaphrodites, like A. rhodensis males, produce only two rather than four functional sperm during meiosis [ 6 , 7 ].…”

“…During anaphase II, the lagging X chromatid invariably ends up in the functional male sperm, whereas the other chromosomal complement is discarded in a “residual body” ( Figure 2 ) [ 6 , 7 ]. In XX A. rhodensis hermaphrodites, clusters of synchronously dividing spermatocytes arise from discrete clusters of spermatogonial cells [ 15 ]. Analysis of 520 hermaphrodite gonads yielded 16 clusters with anaphase II stage spermatocytes.…”

“…Intriguingly, selfing hermaphrodites regularly produce more males early in their broods [ 8 ], suggesting that the choice of the division pattern is developmentally regulated. Furthermore, because sperm within the hermaphrodite germline are produced in spermatogonial clusters [ 15 ], it may be that different clusters produce sperm with different X chromosome complements.…”

“…To obtain A. rhodensis adults of specific sexes, A. rhodensis hermaphrodites were isolated by selecting dauer larvae [ 9 ]. Males and females were isolated from early broods of A. rhodensis hermaphrodites [ 8 ] and the gonads of females were secondarily verified by the absence of spermatogonia [ 15 ].…”

Liposome-based transfection enhances RNAi and CRISPR-mediated mutagenesis in non-model nematode systems

“…Moreover, A. rhodensis males produce haploid cells that are discarded as female polar bodies 11,12 , and sex determination is influenced by the mother’s age 13 . A. rhodensis is unique among hermaphroditic nematodes in that two distinct groups of stem cells in the hermaphrodite germline produce either sperm or oocytes 14 . Thus, Auanema is a suitable model for basic cell biology, genetics, and evolution.…”