Postmating–prezygotic isolation between two allopatric populations of Drosophila montana: fertilisation success differs under sperm competition

Abstract

Postmating but prezygotic (PMPZ) interactions are increasingly recognized as a potentially important early‐stage barrier in the evolution of reproductive isolation. A recent study described a potential example between populations of the same species: single matings between Drosophila montana populations resulted in differential fertilisation success because of the inability of sperm from one population (Vancouver) to penetrate the eggs of the other population (Colorado). As the natural mating system of D. montana is polyandrous (females remate rapidly), we set up double matings of all possible crosses between the same populations to test whether competitive effects between ejaculates influence this PMPZ isolation. We measured premating isolation in no‐choice tests, female fecundity, fertility and egg‐to‐adult viability after single and double matings as well as second‐male paternity success (P2). Surprisingly, we found no PMPZ reproductive isolation between the two populations under a competitive setting, indicating no difficulty of sperm from Vancouver males to fertilize Colorado eggs after double matings. While there were subtle differences in how P2 changed over time, suggesting that Vancouver males’ sperm are somewhat less competitive in a first‐male role within Colorado females, these effects did not translate into differences in overall P2. Fertilisation success can thus differ dramatically between competitive and noncompetitive conditions, perhaps because the males that mate second produce higher quality ejaculates in response to sperm competition. We suggest that unlike in more divergent species comparisons, where sperm competition typically increases reproductive isolation, ejaculate tailoring can reduce the potential for PMPZ isolation when recently diverged populations interbreed.

Persistent postmating, prezygotic reproductive isolation between populations

“…Such asymmetries generate predictions to test in future research; if postmating sexual selection is stronger in the Colorado population, then Colorado males should have more competitive and/or otherwise preferred ejaculates than Vancouver males. While a previous study did not find PMPZ isolation between these two populations under a sperm competitive scenario (Ala‐Honkola, Ritchie, & Veltsos, ), the Colorado population they used had very low within‐population fertilization success and subsequently went extinct in the laboratory, suggesting some kind of inbreeding depression. Our current research shows recurrent, strong PMPZ isolation between these populations that is not dependent on a particular collection from a particular time and we conclude that PMPZ isolation occurs consistently between these populations (see also Moorhead, ).…”

The Rate of Evolution of Postmating-Prezygotic Reproductive Isolation in Drosophila

“…In animals, less is known about the evolution and prevalence of PMPZ RIMs compared with premating or postzygotic mechanisms ( Servedio 2004 ; Pitnick et al 2009 but see Markow 1997 ; Civetta and Clark 2000 ; Birkhead and Pizzari 2002 ; Civetta et al 2008 ; Sweigart 2010 ; Jennings et al 2011 ; Ahmed-Braimah and McAllister 2012 ; Larson et al 2012 ; Ahmed-Braimah 2016 ; Ala‐Honkola et al 2016 ; Miller and Pitnick 2002 , Pitnick et al 2001 , Sagga and Civetta 2011 ). This is surprising because this type of barrier seems to be common (e.g., Fricke and Arnqvist 2004 ; Mendelson et al 2007 ; Dopman et al 2010 ).…”

Rate and success of study replication in ecology and evolution

“…The n = 10 replication studies (including Holman (2014) but not including Amos (2009); Cath et al, (2008)) yielded n = 11 effects ( n = 2 effects in the Jennings, Snook & Hoikkala (2014)/Ala-Honkola, Ritchie & Veltsos (2016) replicate pair). Of these 11 effects, the replication authors concluded that their replication was successful in 36% of cases.…”

“…Of these 11 effects, the replication authors concluded that their replication was successful in 36% of cases. I was able to calculate an effect size for both the original and replicate in only three cases; n = 1 for the Pasukonis et al (2016)/Ringler et al (2013) pair and n = 2 for the Jennings, Snook & Hoikkala (2014)/Ala-Honkola, Ritchie & Veltsos (2016) pair. In the other eight cases, both the replication and original were experiments with qualitative outcomes and did not record quantitative data, or either the original study or replication did not provide the data required to calculate an effect size (Table 1).…”