Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 103

necessary to evolve. Counter to this, remember that sexual reproduction wards off
the chance of mutations affecting the function and structure of multicellulars by join-
ing the haploid genomes of different individuals in each generation.
Epigenetic Determination of the Primary Sex in Vertebrates
Sex determination is considered a textbook example of the genetic determination of
sex in metazoans. In mammals, including humans, sex is determined by combining
two types of sex chromosomes, X and Y. Combination XX induces the development
of female individuals and XY produces males. Females containing two identical sex
chromosomes are homogametic, and males with two different sex chromosomes are
heterogametic. The opposite is observed in birds, where males are homogametic
(ZZ) and females heterogametic (ZW).
The prevailing opinion is that “sex genes” are first expressed in the gonads, and
the secretion of sex hormones by gonads determines the differentiation of male and
female phenotypes. Accordingly, the signal cascade of genetic determination of sex
looks as follows ( Crews, 1993 ):
Zygote→gonad determining genes→gonad formation→hormones→sexual differentiation
of phenotype.
However, in the past decade, evidence is accumulating to unambiguously dem-
onstrate that differentiation of male and female phenotypes is epigenetically, and
through gonads, determined in many species and higher taxa.
Epigenetic Determination of Secondary Sex in Fish
The temperature during early development causes changes in the normal sex ratio in
many fish species ( Blázquez et al., 2009 ). This is related to changes in the amount
and type of aromatase the brain produces in response to the incubation temperature.
Contrary to the earlier belief that the sexual differentiation of gonads and secre-
tion of gonadal hormones induced sexual differentiation in the brain in fish, ample
evidence shows that sexual differentiation in the brain of fish embryos starts before
gonadal differentiation, especially with respect to the expression of brain aro-
matase ( Vizziano-Cantonnet et al., 2011 ); hence, it is more likely that brain signals
induce sexual differentiation of gonads. The brain and gonads of the fish embryo
express both types of aromatase, CYP19A and CYP19B, but their expression in
the brain occurs before the sexual differentiation of gonads, and the expression
of the CYP19B is higher in the brain ( Matsuoka et al., 2006; Vizziano-Cantonnet
et al., 2011 ).
The above evidence shows that, contrary to conventional wisdom, it is not the
gonadal hormone secretion that determines the sexual differentiation of the brain.
The reverse seems to be true: sexual differentiation of the brain determines the
sexual differentiation of gonads and the animal phenotype in vertebrates. As Scott
Gilbert puts it, “sex appears truly to reside in the brain” ( Gilbert, 2005 ).