Allozymic and cytological evidence for hemiclonal, all‐paternal, and mosaic offspring of the hybridogenetic stick insect Bacillus rossius‐grandii grandii

Abstract

Bacillus hybridogens represent the first example of hemiclonal reproduction among invertebrates and they were analyzed in the present study for comparison with vertebrate hybridogenetic complexes (Poeciliopsis and Rana). Experimental crosses of southeastern hybridogenetic B. rossius‐grandii grandii females with both the parental species and hybrid males were carried out to test the reproductive potential of hybridogens. Allozyme and chromosome markers indicated that hemiclonal females actually reproduced in a variety of ways. Hybridogenesis was the most common and the rossius unassorted haploset was passed to the progeny while the grandii haploset was discarded during meiosis and replaced by that of the fathering male. Andro‐genesis, which was much rarer, led to the production of two types of all‐paternal offspring: unipa‐rental progeny of both sexes and all‐male progeny with the same allozyme and chromosome structure as their father. Mosaics of two kinds were also produced: the “h + h” type, with two different hybridogenetic cell lines; and the “h + a” type, with one hybridogenetic and one androgenetic cell line. Mosaics evolved into intersexes when their cell lines were sexually discordant. Among the evolutionary outcomes of this wide array of reproductive ways seems to be a shift from hemiclonality to clonality. This possibility is supported by the well‐established parthenogen B. whitei and the occasional production of rare invariant specimens from the northwestern hybridogenetic form B. rossius‐grandii benazzii. Bacillus and other unisexuals provide evidence of numerous reproductive interactions between hybrids and their bisexual relatives: the occurrence of such interactions may play an important role in the persistence, diversification and evolution of unisexuals. © 1995 Wiley‐Liss, Inc.

Androgenesis: where males hijack eggs to clone themselves

“…In addition to the elimination of the paternal genome from all oocytes, the maternal genome is eliminated in up to 20% of a hybrid female's eggs, resulting in non-nucleate eggs [14]. It is not known whether the elimination (or inactivation) of the maternal genome is controlled by the sperm, or whether it may happen accidentally as a consequence of an imperfect mechanism underlying the elimination of the paternal genome.…”

“…As we explain in the following sections, whether and how diploidization occurs, and under which form of sex determination, greatly affects the predicted fate of androgenetic lineages. While androgenesis is rare in nature, it is 'normal' in some natural populations, including clams of the genus Corbicula [3][4][5], a conifer [6][7][8], a few ants [9][10][11][12] and stick insects [13][14][15][16], and most likely an Australian carp gudgeon [17] (table 1). Androgenesis is also seen sporadically in some species, particularly hymenopterans (ants, bees and wasps), and in some monocots, dicots and gymnosperms (table 1).…”

Current reproductive isolation between ancestors of natural hybrids in Bacillus stick insects (Insecta: Phasmatodea)

“…Speciation through hybridization, fertility and demographic advantages of unisexual hybrids are topics that evolutionary biologists have tackled recently, using morphological, genetic and molecular approaches (reviews in Bullini, 1985Bullini, , 1994Dawley & Bogart, 1989;Bullini & Nascetti, 1990;Scali et al, 1995). Interspecific hybrids arise when frequent heterospecific matings lead to the production of zygotes that occasionally overcome developmental constraints.…”

“…hybridogenesis, parthenogenesis, unisexual & Bogart, 1989;Bullini & Nascetti, 1990;Scali et al, 1995). Although gynogenetic and parthenogenetic hybrids are thought to be genetically isolated from their ancestors, reproductive interactions between them occur allowing hybrid genome evolution by addition of subgenomic fractions or of whole haplosets (Cimino, 1972;Quattro et al, 1992;Scali et al, 1995;Schartl et aL, 1995;Tinti & Scali, 1996).…”

“…Although gynogenetic and parthenogenetic hybrids are thought to be genetically isolated from their ancestors, reproductive interactions between them occur allowing hybrid genome evolution by addition of subgenomic fractions or of whole haplosets (Cimino, 1972;Quattro et al, 1992;Scali et al, 1995;Schartl et aL, 1995;Tinti & Scali, 1996). The very rare hybridogenetic hybrids produce reduced eggs by a modified gametogenesis allowing the transmission to the offspring of an unrecombined parental genome (hemiclone), the other one being completely discarded during oogenesis and sexually provided anew by host fathering males (reviews in Dawley & Bogart, 1989;Scali et a!., 1995 Mantovani & Scali, 1993); B. grandii is strictly bisexual, whereas B. rossius has also all-female, facultatively parthenogenetic populations. Bacillus rossius-grandii benazzii and B. rossius-grandii grandii are hybridogenetic strains consisting of allodiploid females (2n = 35).…”