The seemingly shared ability of independently evolved Extavo
The seemingly shared ability of independently evolved (Extavour and Akam, 2003; Ewen-Campen et al., 2010) maternal germ plasm regulators to engage multiple proteins and RNA partners provide new insight and testable models for mechanisms by which germ plasm assembly, mitochondrial recruitment and Balbiani body formation are regulated, including additional support for the possibility that a key aspect of regulation could include differential association with a unique repertoires of RNAs, directly via RNA binding domain interactions and either directly or indirectly through interactions with RNA binding proteins (Heim et al., 2014; Boke et al., 2016; Jeske et al., 2015). Consistent with the notion of temporally distinct regulation and activities, Boke and colleagues found that XVelo could only promote assembly of amyloid-like networks in oocytes, but not in eggs. It is intriguing that the ability to recruit mitochondria and RNA to the matrix based on the evidence so far appear to be linked in these vertebrate oocytes, suggesting a coordinated mechanism, or that recruitment of one is prerequisite for recruitment of the other, for example one could imagine that an RNA encoding a protein that is required to select or anchor mitochondria might be specifically translated or localized within the Balbiani body. In Drosophila milton mutants recruitment of mitochondria to the Balbiani body is uncoupled from RNA localization there (Cox and Spradling, 2006). In Drosophila RNA labeling and tracking experiments indicates that the germ plasm map3k8 RNAs become entrapped and concentrated in the localized germ plasm owing to their affinity for germ plasm proteins (Little et al., 2015; Trcek et al., 2015). In Drosophila, tudor mutants that disrupt an RNA binding protein that acts downsteam of oskar, mitochondria associate with pole granules; however, recruitment or transfer of mitochondrial ribosomal RNAs to the surface of polar granules (germ plasm) is disrupted (Ding et al., 1994; Kashikawa et al., 1999; Amikura et al., 2001a,b). Animals that don\'t use maternal germ plasm, including humans, have Balbiani bodies and genes like oskar or bucky ball XVelo. This raises the possibility that nonmembrane bound compartments formed by self-assembling proteins like XVelo, Bucky ball and Oskar originated for mitochondrial entrapment and selection, and were coopted or coupled to amass, chaperone and coordinately regulate cohorts of RNA, like the germ plasm through their interactions with RNA binding proteins, which have been shown to undergo dynamic phase transitions through concentration dependent interactions with their RNA and protein partners (Fig. 4) (Brangwynne et al., 2009; Brangwynne, 2013; Lin et al., 2015; Zhang et al., 2015). Compared to Velo or Bucky Ball, much more is understood about the complex regulation of Oskar, which includes regulation by post-transcriptional and posttranslational mechanisms, such as phosphorylation and ubiquitination (Fig. 3) (reviewed in Lehmann, 2016). Further analysis of XVelo/Bucky ball functional domains and the genes regulating Balbiani body development will be required to resolve these questions. Important lines of investigation ongoing and for the future include further defining the binding partners of the vertebrate specific Velo and Bucky ball, and applying genetic and biochemical approaches to tease out the mechanisms that regulate, both spatially and temporally, the various RNA and protein associations of these proteins in the context of their functional consequences to germ cell development and fertility. The prior observations that zebrafish bucky ball mutants and transgenics have ample mitochondria and, at least in terms of oocyte growth are indistinguishable from normal oocytes provides evidence that sorting to the Balbiani body is not required for mitochondria amplification in zebrafish (Marlow and Mullins, 2008; Heim et al., 2014). However, whether or not the mitochondria of zebrafish bucky ball mutants are sufficient for growth but are less fit, or if bucky ball mutant oocytes are more prone to heteroplasmy in the absence of Balbiani body enrichment, which might be expected if the Balbiani body serves to select the mitochondria as part of an ancient oocyte bottleneck, has yet to be determined. Moreover, the question of whether the mitochondria that colocalize with the germ plasm in the Balbiani body are later selectively sorted to the primoridial germ cells of the embryo remains to be addressed, but should be feasible with modern genome editing technologies.