Possible Third Step Preventing Conjugation between Different Species of Blepharisma

Abstract

In the genus Blepharisma, reproductive isolation between different species appears to be established at least by two barriers: (1) a mating pheromone, i.e., gamone 1, and (2) a factor involved in pair formation. Using four species, we experimentally investigated other potential barriers to interspecific conjugation in Blepharisma, as well as the first and second barriers. Cell-free fluid from type I cells (CFF1) of B. americanum had no effect on B. undulans, B. japonicum, or B. stoltei. Type II cells of B. americanum responded to CFF1 from B. americanum but not to CFF1 from B. undulans, B. japonicum, or B. stoltei. Gamone 1, therefore, seems to be the first reproductive barrier (with the inclusion of B. americanum species [megakaryotype 3]) as reported previously. In pretreated cells with complementary gamones in B. undulans and B. americanum, inter-species pair formation was rare, but pair formation between B. americanum and B. japonicum and between B. americanum and B. stoltei occurred at relatively high frequency. Most of the inter-species B. americanum–B. stoltei pairs underwent nuclear changes specific to conjugation. No significant difference was observed between the intra- and inter-species pairs over the time course of the nuclear changes, but the percentage of abnormal cells was higher in inter-species pairs than in intra-species pairs, and no progenies were produced by inter-species pairs. These results suggest a third barrier or step, in addition to the first and second ones, in nuclear changes after pair formation that prevents interspecific conjugation in Blepharisma.

Mating Pheromone (Gamone 1) in Blepharisma: A Glycoprotein Responsible for Species Diversity in Unicellular Organisms (Alveolata, Ciliophora)

“…Our research on interspecies conjugation revealed at least three essential steps for completion: (1) the gamone 1-mediated step (inducing mating pairs in type II cells), (2) assessing the formation of mating pairs by activated cells, and (3) producing offspring [ 23 , 24 ]. Essentially, gamone 1 exhibits species specificity; even within the same megakaryotype, it does not function across species (e.g., B. americanum and B. musculus , both belonging to Mk3, do not interact with each other) [ 22 , 23 , 24 ]. This implies that reproductive isolation essentially occurs at Step 1.…”

“…However, a situation might occur where different species cohabit the same location, suggesting the potential for type II cells activated by the same species of gamone 1 to encounter activated type I cells and form mating pairs. We have reported on interspecies mating pair formation among artificially activated cells [ 23 , 24 ]. Yet, even when cells treated with gamone 1 of the same species are brought together, interspecies mating pairs are scarcely formed (e.g., between B. undulans (Mk2) and B. japonicum (Mk4), and between B. undulans (Mk2) and B. americanum (Mk3)), while in some cases, interspecies mating pairs are formed (e.g., between B. americanum (Mk3) and B. japonicum (Mk4) or between B. americanum (Mk3) and B. stoltei (Mk4).…”

“…This indicates that while interspecies pair formation is feasible between certain species, it remains unattainable between others at Step 2. Moreover, even in cases where interspecies mating pairs between B. americanum (Mk3) and B. stoltei (Mk4) occur, no offspring are produced between species (i.e., infertility) [ 24 ]. In this case, while Step 1 and Step 2 were successfully passed, Step 3 was not.…”

“…Gamone 2 is an entirely different substance, an amino acid derivative identified as 3-(2′-formylamino-5′-hydroxybenzoyl) lactate [ 19 ]. Gamone 1 reportedly has a species-specific effect in the induction of conjugation, and gamone 2 is common among Blepharisma species [ 22 , 23 , 24 ]. The phenomenon of entirely different substances inducing conjugation in type I and type II is intriguing, as there are no examples in other ciliates.…”