Follicle Stem Cells (FSCs) in the Drosophila ovary; a critique of published studies defining the number, location and behavior of FSCs

Abstract

SummaryA paper by Reilein et al., (2017) presented several key new insights into the behavior of adult Follicle Stem Cells (FSCs) in the Drosophila ovary, including overwhelming evidence that each ovariole hosts a large number of FSCs (about 14-16) maintained by population asymmetry (Reilein et al., 2017), rather than just two FSCs, dividing with largely individually asymmetric outcomes, as originally proposed (Margolis and Spradling, 1995; Nystul and Spradling, 2007). Here we provide further discussion asserting the merits of the conclusions of Reilein et al., (2017) and the deficiencies in the contrary assertions recently presented by Fadiga and Nystul (Fadiga and Nystul, 2019). The principles that we discuss here, particularly with regard to lineage tracing and population asymmetry, are common to the investigation of most types of adult stem cell and should therefore be instructive and of interest to investigators studying any type of adult stem cell. The improved understanding of FSC numbers, location and behavior afforded by Reilein et al., (2017) and Reilein et al., (2018) can only provide a firm foundation for future progress once they are widely appreciated and seen to be resistant to challenge, as described in detail here.

Determination of expression profiles for Drosophila ovarian Follicle Stem Cells (FSCs) using single-cell RNA sequencing

“…The assignment of a group of scRNA expression profiles to FSCs is not precise for a number of reasons, including imperfect and limited depth of RNA profiles, latitude in spatial correspondence according to RNA in situs, and the systematic limitation that FSCs become ECs or FCs within a few hours and therefore inevitably have strong similarities to those neighboring derivatives. Nevertheless, the results of this study have the unique virtue that the approximately-defined FSC group of cells is in accord with current understanding of FSC numbers and locations determined by thorough functional analyses (2,9,13,17,(58)(59)(60). FSC behavior is guided by a number of external signals but with prominent roles for two graded signaling pathways, Wnt and JAK-STAT (13,17).…”

Opposing JAK-STAT and Wnt signaling gradients define a stem cell domain by regulating differentiation at two borders

“…A key aspect of this revision was the realization that earlier studies had implicitly assumed that FSCs are long-lived and maintained by single-cell asymmetry, leading to the exclusion of the majority of FSC lineages from all analyses, necessarily leading to substantial under-estimation of FSC numbers, over-estimation of average FSC longevity and obscuring the evidence that FSCs are in fact maintained by population asymmetry. Our current picture of FSCs is supported by extensive direct evidence; recent claims supporting the original conception of FSCs ( Fadiga and Nystul, 2019 ) were addressed, and in our opinion, refuted comprehensively ( Kalderon, 2020 ). Nevertheless, additional evidence can always contradict, confirm or refine an existing model.…”

Extracellular spreading of Wingless is required for Drosophila oogenesis

“…Fortunately, we were able to use the alternative approach of direct visualization of mitotic cells with anti-pH3 (Fig 3A and 3B), and we analyzed their frequency in the germarium. Currently it is thought that there are three layers or rings of FSCs, extending anteriorly from the 2a/2b boundary, with proliferation occurring mostly in the posterior ring at the boundary of bright Fas3 staining [25][26][27]. This understanding of FSCs is at odds with an older model, which held there were only two FSCs [28,29], but even in this model, FSC proliferation is at the border between regions 2a and 2b, at the boundary of bright Fas3 antibody staining [29][30][31][32].…”