Evolutionary conservation of lampbrush-like loops in drosophilids

Abstract

Background: Loopin-1 is an abundant, male germ line specific protein of Drosophila melanogaster. The polyclonal antibody T53-F1 specifically recognizes Loopin-1 and enables its visualization on the Y-chromosome lampbrush-like loop named kl-3 during primary spermatocyte development, as well as on sperm tails. In order to test lampbrush-like loop evolutionary conservation, extensive phase-contrast microscopy and immunostaining with T53-F1 antibody was performed in other drosophilids scattered along their genealogical tree.

Results: In the male germ line of all species tested there are cells showing giant nuclei and intranuclear structures similar to those of Drosophila melanogaster primary spermatocytes. Moreover, the antibody T53-F1 recognizes intranuclear structures in primary spermatocytes of all drosophilids analyzed. Interestingly, the extent and conformation of the staining pattern is species-specific. In addition, the intense staining of sperm tails in all species suggests that the terminal localization of Loopin-1 and its orthologues is conserved. A comparison of these cytological data and the data coming from the literature about sperm length, amount of sperm tail entering the egg during fertilization, shape and extent of both loops and primary spermatocyte nuclei, seems to exclude direct relationships among these parameters.

Conclusion: Taken together, the data reported strongly suggest that lampbrush-like loops are a conserved feature of primary spermatocyte nuclei in many, if not all, drosophilids. Moreover, the conserved pattern of the T53-F1 immunostaining indicates that a Loopin-1-like protein is present in all the species analyzed, whose localization on lampbrush-like loops and sperm tails during spermatogenesis is evolutionary conserved.

Figure 1

Genealogical trees of the drosophilids analyzed in the present work. a: general phylogeny; b: partial phylogeny of the melanogaster subgroup, an enlargement of the simulans/melanogaster branch of the upper tree. Numbers in panel a indicate how many million years ago the species diverged. The common ancestor of all these species is supposed to have lived some 60 million years ago; the divergence between D. melanogaster and D. tessieri probably occurred between 10 and 15 million years ago. The upper tree is partially taken and modified from [13]. The lower tree is partially taken and modified from [48].

Figure 2

T53-F1 immunostaining of primary spermatocyte nuclei of drosophilids. Note that in all species it is possible to recognize intranuclear structures specifically decorated by the antibody. For all species, the first image is a phase-contrast micrograph, the second shows the corresponding immunostaining. a-a': D. acanthoptera. b-b': D. americana. c-c': D. bifurca. d-d': D. funebris. e-e': D. littoralis. f-f': D. mauritiana. g-g': D. mercatorum. h-h': D. pseudoobscura. i-i': D. sechellia. j-j': D. tessieri. k-k': D. simulans. l-l': D. texana. m-m': D. yakuba. Bars: 10 μm.