論文の公開元へPersistence of a highly monoandrous mating system despite an extremely male-biased operational sex ratio in the firefly squid Watasenia scintillans
概要
Broadly, the points at which females choose males as mating partners, referred to as AQ3 female choice, call special attention to
evolutionary biologists (Emlen and Oring 1977 ; Eberhard 1996 ; Rosenthal and Ryan 2022 ). However, how females decide the
number of males to mate, especially single or multiple males, remains largely unknown (Jennions and Petrie 2000 ; Kvarnemo
2018 ). In theory, mate number is disproportionally correlated with offspring number in females (Bateman 1948 ); however,
polyandry (female mating with multiple males) is prevalent in diverse taxa (Taylor et al. 2014 ) owning to direct and indirect
AQ4 benefits gained from multiple mates (Reynolds 1996 ; Arnqvist and Nilsson 2000 ; Firman 2011 ; Matsumura et al. 2021 ). It is
generally considered that the mode of reproduction, either monoandry (female mating with a male) or polyandry, is attributed to
each species as a fixed trait (Whiteman and Cote 2004 ; Hughes et al. 2008 ; Davies and Gardner 2018 ; Young et al. 2019 ).
However, monoandry (or monogamy) is considered to have been adopted to maximize reproductive success for one or both sexes as
the consequences of biparental care (Tumulty et al. 2014) and severe constraints on the accessibility and availability of reproductive
or environmental resources (Komers and Brotherton 1997 ; Stockley 1997 ). Therefore, it is often observed that the choice of
reproductive mode is determined conditionally by environmental, intrasexual, or intersexual contexts, and hence, differently in
populations (Uller and Olsson 2008 ; Brown et al. 2010 ).
In this case, the operational sex ratio (OSR), the ratio of males to females who are ready to mate at any one time (Emlen and Oring
1977 ), may influence the reproductive mode decision in a given population (Arnqvist and Nilsson 2000 ; Kokko and Jennions 2008 ;
Janicke and Morrow 2018 ), but see (Head et al. 2008 ; Plesnar-Bielak et al. 2020 ). If the OSR skews toward one sex, the intensity
of competition for mates increases (Trivers 1972 ), but see (Klug et al. 2010 ), resulting in altered mating behaviors, such as either
an increase or decrease in competitive aggression, courtship, pre- or postcopulatory mate guard, and frequency or duration of
copulation (Kvarnemo and Ahnesjo 1996 ; Weir et al. 2011 ). We assumed that in species without aggression, courtship, mate
guarding and parental care, a possible behavioral change that may occur in response to increased male-biased OSR is a higher
frequency of copulation with higher levels of polyandry (Lode et al. 2004 ; Naud et al. 2004 ). In this regard, the firefly squid
Watasenia scintillans offers a model system suitable for testing this hypothesis because (1) they are semelparous and unlikely to
engage courtship and mate guarding due to a considerable length of time gap between copulation and spawning (Hayashi 1995 ); (2)
females are approximately 5% polyandrous (Sato et al. 2020 ), allowing us to detect any small increase in the rate of polyandry; (3)
adult sex ratio changes greatly from male-biased to female-biased (Sato et al. 2020 ), hence OSR may also change accordingly,
during the mating season in the wild; and (4) the level of polyandry can be estimated at a large population scale with a newly
developed anatomical method (described in the present study). In addition to this question concerning the possible impact of OSR
on monoandry-polyandry decisions, we are intrigued by the highly monoandrous reproductive mode in W. scintillans, because most
cephalopods are thought to adopt a polyandrous mating strategy, as suggested by field observations and paternity tests with DNA
fingerprinting (Hanlon et al. 1997 ; Naud et al. 2004 , 2016 ; Iwata et al. 2005 ; Sato et al. 2014 ). In the wild population, males
mature in advance and await females’ receptivity to mate. Most females copulate once at sexually premature stage during the short
mating period of ∼3 weeks and store male-delivered sperm sacs (spermatangia) for the rest of their life for no longer than 3 months
(Sato et al. 2020 ). Microsatellite DNA-based paternity analysis revealed behavioral monogamy (all sperm sacs stored in the female
were transferred by a single male), and as the consequence, genetic monoandry (all eggs in the same clutch, though investigated
with limited sample size, were fathered by a single male) (Sato et al. 2020 ). From these data, we speculated two possibilities: (1)
polyandry is an alternative common strategy that occurs if certain conditions are met, or (2) polyandry occurs as a consequence of
unusual coincidences or pathological actions. ...
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