3. Results
3.1. Sperm plasma membrane integrity (PMI,
SYBR-14/propidium iodide assay)
A characteristic dot plot on SCC vs FSC which
classified as sperm are indicated is shown in Fig. 1a,
whereas region of analysis between SYBR-14 versus PI
is depicted in Fig. 1b. The percentage of spermatozoa
with intact plasma membrane (alive, SYBR-14+/PI)
was significantly (P < 0.001) higher in winter (54.6%)
than in the rainy season (43.5%) or summer (46.7%)
(Table 3). The average percentage of moribund
spermatozoa (SYBR-14+/PI+) differed significantly
(P < 0.01) only between winter and summer. Consequently,
the average of dead spermatozoa (SYBR-14/
PI+) was significantly (P < 0.01) lower in winter
(37.5%) than in the rainy season (47.6%) or summer
(43.6%). The average percentage of PMI (SYBR-14+/
PI) and dead spermatozoa (SYBR-14/PI+) was
affected by the year of semen collection/processing
(P < 0.01).
3.2. Sperm plasma membrane stability (PMS,
Annexin-V/propidium iodide assay)
A typical two-dimensional dot plot from a flow
cytometric profile of frozen-thawed sperm labeled with
Annexin V and PI is shown in Fig. 2. The average
percentage of viable spermatozoa (AN/PI) with an
intact plasma membrane was also higher in winter
(55.7%) than in the rainy season (40.1%) and summer
(53.0%), but differed significantly (P < 0.001) only
between winter and the rainy season (Table 3). The
percentage of spermatozoa with an impaired yet intact
plasma membrane (AN+/PI) was apparently higher in
winter than during the rainy season and summer (ns),
with the average percentage of dead spermatozoa
(AN+/PI+) being significantly (P < 0.001) higher in the
rainy season (52.8%) than in winter (36.8%) or summer
(40.5%). The average percentages of both viable
spermatozoa (AN/PI) and dead spermatozoa
1428 S. Koonjaenak et al. / Theriogenology 67 (2007) 1424–1435
Table 2
Variables defining the seasons which the comparisons in the present study are based on (mean standard deviation (S.D.) of the years included in the
study, 1980–1989, and 2003, 2004 and 2005)
Season Temperature (mean maximum, 8C) Rainfall (mean maximum, mm) Humidity (mean maximum, %)
Rainy season 32.4 1.0 a 47.8 29.1 a 93.0 4.3 a
Winter 31.4 1.8 a 5.5 12.8 b 89.7 4.4 a
Summer 34.7 1.7 b 48.7 34.9 a 84.5 7.5 b
a–bDifferent superscripts indicate significant differences within variables (P < 0.05).
Fig. 1. (a) Dot-plot of FSC vs. SCC from flow cytometry where events
classified as spermatozoa are indicated. (b) Dot-plot of SYBR-14
(FL1) vs. PI (FL3) fluorescence. Region of analysis indicated in the
plot.